feat(auth): rebuild the sign-in surfaces on the shared sign-card contract

The login/register page and the auth modal had drifted into a parallel
system: floating-input fields, a sign-page-card shell and a bespoke
jio-auth-modal wrapper that none of the theme's own sign CSS targeted.
The reference card rules (.sign, .sign-img, .sign-content, .line-form-input,
.scale-placeholder, .social_loginbar, .title-h-left, #u_sign .modal-dialog,
.qrcode-signin-container) have been in main.css all along, so the fix is to
render the reference DOM instead of re-styling a second one.

auth-content.php now emits one .sign.jio-panel.blur-bg.relative card with a
.tab-content of #tab-sign-in / #tab-sign-up / #tab-resetpassword /
#tab-qrcode-signin / #tab-bind panes. Each pane opens with a
.title-h-left.fa-2x heading plus a .muted-color.px12 link, fields are
.relative.line-form.mb10 > .line-form-input + .scale-placeholder +
.line-form-line, the verification trigger is the .yztx.abs-right captchsubmit
button, and submits are .but.radius.jb-blue|jb-green.padding-lg.btn-block.
Social entries come from the shared render_social_buttons() separator +
.social_loginbar, and the OAuth bind pane uses the oauth-data-box avatar card.

The modal shell is #u_sign.modal > .modal-dialog > .sign-content >
.sign-img.absolute.hide-sm + .sign, and the page shell is the reference
.container.sign-page > .row.sign-row.gutters-5 > .col-md-6(.col-md-offset-6/
.col-md-offset-3).sign-col with a .fixed.lazyload background and a blurred
fixed footer. auth.js drives .in instead of .is-open, toggles the new
password/signin-mode/invite-gate controls and no longer touches the retired
markup. The page also drops two footer-hook removals that targeted functions
which no longer exist, and opts out of the fixed-nav offset.

The sign page's custom theme setting now actually applies: Router answers
jio_forced_theme_mode, jioData.themeForced wins over the stored cookie in
theme.js, theme-toggle positions are suppressed, and the page marks html/body
dark or white accordingly.

sign.css shrinks from ~114 lines of bespoke component styling to a thin
positioning layer, and the retired classes are gone rather than kept.

Verification: php -l on every changed PHP file, node --check on both JS files,
node --test across widget-system, navigation and front-submit (873 passing),
and tools/audit-psr4-paths.php / audit-admin-contracts.php both at 0 errors.
assets/js/runtime/main.min.js was rebuilt because theme.js gained the forced
theme branch.

src/Core/ScriptsPart1c.php

@@ -463,6 +463,8 @@ trait ScriptsPart1c {
             'siteUrl'               => home_url('/'),
             'themeUrl'              => JIO_URI,
             'themeMode'             => class_exists('\Jio\Settings\Settings') ? \Jio\Settings\Settings::theme_mode_js_value() : 'auto',
+            // 登录页「自定义主题」：强制模式优先于用户保存的 cookie。
+            'themeForced'           => (string) apply_filters('jio_forced_theme_mode', ''),
             'debug'                 => defined('WP_DEBUG') && WP_DEBUG,
             'debugLog'              => defined('WP_DEBUG') && WP_DEBUG && defined('WP_DEBUG_LOG') && WP_DEBUG_LOG,
             'postPaginationMode'    => class_exists('\Jio\Settings\Settings') ? \Jio\Settings\Settings::get_post_pagination_mode() : 'load_more',