refactor(creator-studio): pick the cover from the toolbar image dialog

The cover row carried its own hidden file input and upload call, so authors
had two different image flows in one page.

- 添加封面 now opens window.JioSubmitMedia with type=image, the same dialog
  the toolbar's 图片 button uses (mine / upload / external link).
- The picked image is stored as the full-size URL plus its attachment id,
  matching FrontSubmitFeatured's image branch, which re-checks ownership
  and sets the post thumbnail.
- Removes the now-unused uploadCover path, apiUpload/nonce imports, and the
  .jio-creator__cover-input rule.

Contract added; 887 widget-system tests pass.

assets/creator-studio/build/index.css

@@ -1 +1 @@
-.jio-page-submit{background:#f6f6f6}body.jio-page-submit.has-static-nav,body.jio-page-submit.nav-fixed{padding-top:0!important}.jio-page-submit .float-btn,.jio-page-submit .footer,.jio-page-submit .footer-tabbar,.jio-page-submit .header,.jio-page-submit .jio-float-buttons{display:none!important}.jio-page-submit #main-content.container{max-width:none;padding:0}#jio-creator-studio.is-ready{display:block}.jio-creator{--jc-ink:#2b2b2b;--jc-ink-soft:#7d8683;--jc-paper:#f6f6f6;--jc-line:#e6e9e8;--jc-accent:#267665;--jc-accent-strong:#1d5f52;--jc-warn:#b4573f;--jc-top:112px;--jc-status:52px;-webkit-font-smoothing:antialiased;background:var(--jc-paper);color:var(--jc-ink);font-family:-apple-system,BlinkMacSystemFont,Segoe UI,Noto Sans SC,PingFang SC,Microsoft YaHei,sans-serif;min-height:100vh;min-height:100dvh;padding-bottom:var(--jc-status);padding-top:var(--jc-top)}.jio-creator *,.jio-creator :after,.jio-creator :before{box-sizing:border-box}.jio-creator button{color:inherit;font:inherit}.jio-creator__top{background:rgba(250,251,250,.98);box-shadow:0 2px 8px rgba(27,46,39,.035);left:0;position:fixed;right:0;top:0;z-index:300}.jio-creator__masthead{align-items:center;display:flex;gap:16px;height:56px;justify-content:space-between;padding:0 clamp(14px,2.4vw,30px)}.jio-creator__masthead-left{align-items:center;display:flex;gap:10px;min-width:0}.jio-creator__back{align-items:center;border-radius:7px;color:var(--jc-ink-soft);display:inline-flex;flex:0 0 auto;height:30px;justify-content:center;text-decoration:none;width:30px}.jio-creator__back:focus-visible,.jio-creator__back:hover{background:#edf3f0;color:var(--jc-accent);outline:0}.jio-creator__brand{align-items:center;color:inherit;display:inline-flex;min-width:0;text-decoration:none}.jio-creator__brand:focus-visible{outline:2px solid rgba(38,118,101,.35);outline-offset:4px}.jio-creator__brand-logo{display:block;height:30px;max-width:168px;-o-object-fit:contain;object-fit:contain;width:auto}.jio-creator__brand-text{color:var(--jc-accent-strong);font-family:Songti SC,"Noto Serif SC","Source Han Serif SC",serif;font-size:17px;font-weight:700;letter-spacing:-.01em;overflow:hidden;text-overflow:ellipsis;white-space:nowrap}.jio-creator__me{align-items:center;border-radius:50%;display:inline-flex;flex:0 0 auto;justify-content:center}.jio-creator__me:focus-visible{outline:2px solid rgba(38,118,101,.5);outline-offset:2px}.jio-creator__me-avatar{background:#eef1f0;border:1px solid rgba(24,39,34,.08);border-radius:50%;display:block;height:32px;-o-object-fit:cover;object-fit:cover;overflow:hidden;width:32px}.jio-creator__me-avatar.is-placeholder{align-items:center;color:#9aa2a0;display:inline-flex;font-size:13px;justify-content:center}.jio-creator__bar{align-items:center;border-bottom:1px solid var(--jc-line);border-top:1px solid var(--jc-line);display:flex;height:56px;justify-content:center;justify-content:safe center;padding:0 clamp(14px,2.4vw,30px)}.jio-creator__toolbar-tools{align-items:center;display:flex;gap:7px;max-width:100%;min-width:0;overflow-x:auto;scrollbar-width:none}.jio-creator__toolbar-tools::-webkit-scrollbar{display:none}.jio-creator__toolbar-group{align-items:center;display:inline-flex;flex:0 0 auto;gap:2px}.jio-creator__toolbar-divider{background:var(--jc-line);flex:0 0 auto;height:22px;width:1px}.jio-creator__tool-btn{align-items:center;background:transparent;border:0;border-radius:6px;color:#5f6664;cursor:pointer;display:inline-flex;flex:0 0 auto;flex-direction:column;font-size:11px;gap:2px;height:46px;justify-content:center;line-height:1.2;min-width:40px;padding:5px 6px;width:auto}.jio-creator__tool-btn i{font-size:14px;line-height:1}.jio-creator__tool-btn:focus-visible:not(:disabled),.jio-creator__tool-btn:hover:not(:disabled){background:#f0f2f1;color:var(--jc-accent-strong);outline:0}.jio-creator__tool-btn.is-active:not(:disabled),.jio-creator__tool-btn.is-open:not(:disabled){background:#e9f1ee;color:var(--jc-accent-strong)}.jio-creator__tool-btn:disabled{color:#c7d0cc;cursor:default}.jio-creator__tool-label{max-width:52px;overflow:hidden;text-overflow:ellipsis;white-space:nowrap}.jio-creator__tool-caret{bottom:4px;color:#b0b9b6;font-size:8px;line-height:1;position:absolute;right:3px}.jio-creator__tool-menu{display:inline-flex;flex:0 0 auto;position:relative}.jio-creator__tool-menu .jio-creator__tool-btn{position:relative}.jio-creator__menu{animation:jio-menu-in .12s ease-out;background:#fff;border:1px solid rgba(24,39,34,.06);border-radius:12px;box-shadow:0 14px 40px rgba(20,32,28,.16),0 2px 6px rgba(20,32,28,.06);display:flex;flex-direction:column;gap:2px;padding:6px;position:fixed;z-index:620}.jio-creator__menu.is-top{animation-name:jio-menu-in-up}@keyframes jio-menu-in{0%{opacity:0;transform:translateY(-4px)}to{opacity:1;transform:translateY(0)}}@keyframes jio-menu-in-up{0%{opacity:0;transform:translateY(4px)}to{opacity:1;transform:translateY(0)}}.jio-creator__menu-item{align-items:center;background:transparent;border:0;border-radius:7px;color:#4Line truncated
+.jio-page-submit{background:#f6f6f6}body.jio-page-submit.has-static-nav,body.jio-page-submit.nav-fixed{padding-top:0!important}.jio-page-submit .float-btn,.jio-page-submit .footer,.jio-page-submit .footer-tabbar,.jio-page-submit .header,.jio-page-submit .jio-float-buttons{display:none!important}.jio-page-submit #main-content.container{max-width:none;padding:0}#jio-creator-studio.is-ready{display:block}.jio-creator{--jc-ink:#2b2b2b;--jc-ink-soft:#7d8683;--jc-paper:#f6f6f6;--jc-line:#e6e9e8;--jc-accent:#267665;--jc-accent-strong:#1d5f52;--jc-warn:#b4573f;--jc-top:112px;--jc-status:52px;-webkit-font-smoothing:antialiased;background:var(--jc-paper);color:var(--jc-ink);font-family:-apple-system,BlinkMacSystemFont,Segoe UI,Noto Sans SC,PingFang SC,Microsoft YaHei,sans-serif;min-height:100vh;min-height:100dvh;padding-bottom:var(--jc-status);padding-top:var(--jc-top)}.jio-creator *,.jio-creator :after,.jio-creator :before{box-sizing:border-box}.jio-creator button{color:inherit;font:inherit}.jio-creator__top{background:rgba(250,251,250,.98);box-shadow:0 2px 8px rgba(27,46,39,.035);left:0;position:fixed;right:0;top:0;z-index:300}.jio-creator__masthead{align-items:center;display:flex;gap:16px;height:56px;justify-content:space-between;padding:0 clamp(14px,2.4vw,30px)}.jio-creator__masthead-left{align-items:center;display:flex;gap:10px;min-width:0}.jio-creator__back{align-items:center;border-radius:7px;color:var(--jc-ink-soft);display:inline-flex;flex:0 0 auto;height:30px;justify-content:center;text-decoration:none;width:30px}.jio-creator__back:focus-visible,.jio-creator__back:hover{background:#edf3f0;color:var(--jc-accent);outline:0}.jio-creator__brand{align-items:center;color:inherit;display:inline-flex;min-width:0;text-decoration:none}.jio-creator__brand:focus-visible{outline:2px solid rgba(38,118,101,.35);outline-offset:4px}.jio-creator__brand-logo{display:block;height:30px;max-width:168px;-o-object-fit:contain;object-fit:contain;width:auto}.jio-creator__brand-text{color:var(--jc-accent-strong);font-family:Songti SC,"Noto Serif SC","Source Han Serif SC",serif;font-size:17px;font-weight:700;letter-spacing:-.01em;overflow:hidden;text-overflow:ellipsis;white-space:nowrap}.jio-creator__me{align-items:center;border-radius:50%;display:inline-flex;flex:0 0 auto;justify-content:center}.jio-creator__me:focus-visible{outline:2px solid rgba(38,118,101,.5);outline-offset:2px}.jio-creator__me-avatar{background:#eef1f0;border:1px solid rgba(24,39,34,.08);border-radius:50%;display:block;height:32px;-o-object-fit:cover;object-fit:cover;overflow:hidden;width:32px}.jio-creator__me-avatar.is-placeholder{align-items:center;color:#9aa2a0;display:inline-flex;font-size:13px;justify-content:center}.jio-creator__bar{align-items:center;border-bottom:1px solid var(--jc-line);border-top:1px solid var(--jc-line);display:flex;height:56px;justify-content:center;justify-content:safe center;padding:0 clamp(14px,2.4vw,30px)}.jio-creator__toolbar-tools{align-items:center;display:flex;gap:7px;max-width:100%;min-width:0;overflow-x:auto;scrollbar-width:none}.jio-creator__toolbar-tools::-webkit-scrollbar{display:none}.jio-creator__toolbar-group{align-items:center;display:inline-flex;flex:0 0 auto;gap:2px}.jio-creator__toolbar-divider{background:var(--jc-line);flex:0 0 auto;height:22px;width:1px}.jio-creator__tool-btn{align-items:center;background:transparent;border:0;border-radius:6px;color:#5f6664;cursor:pointer;display:inline-flex;flex:0 0 auto;flex-direction:column;font-size:11px;gap:2px;height:46px;justify-content:center;line-height:1.2;min-width:40px;padding:5px 6px;width:auto}.jio-creator__tool-btn i{font-size:14px;line-height:1}.jio-creator__tool-btn:focus-visible:not(:disabled),.jio-creator__tool-btn:hover:not(:disabled){background:#f0f2f1;color:var(--jc-accent-strong);outline:0}.jio-creator__tool-btn.is-active:not(:disabled),.jio-creator__tool-btn.is-open:not(:disabled){background:#e9f1ee;color:var(--jc-accent-strong)}.jio-creator__tool-btn:disabled{color:#c7d0cc;cursor:default}.jio-creator__tool-label{max-width:52px;overflow:hidden;text-overflow:ellipsis;white-space:nowrap}.jio-creator__tool-caret{bottom:4px;color:#b0b9b6;font-size:8px;line-height:1;position:absolute;right:3px}.jio-creator__tool-menu{display:inline-flex;flex:0 0 auto;position:relative}.jio-creator__tool-menu .jio-creator__tool-btn{position:relative}.jio-creator__menu{animation:jio-menu-in .12s ease-out;background:#fff;border:1px solid rgba(24,39,34,.06);border-radius:12px;box-shadow:0 14px 40px rgba(20,32,28,.16),0 2px 6px rgba(20,32,28,.06);display:flex;flex-direction:column;gap:2px;padding:6px;position:fixed;z-index:620}.jio-creator__menu.is-top{animation-name:jio-menu-in-up}@keyframes jio-menu-in{0%{opacity:0;transform:translateY(-4px)}to{opacity:1;transform:translateY(0)}}@keyframes jio-menu-in-up{0%{opacity:0;transform:translateY(4px)}to{opacity:1;transform:translateY(0)}}.jio-creator__menu-item{align-items:center;background:transparent;border:0;border-radius:7px;color:#4Line truncated