fix(navigation): let CSS size the nav logo and drop the inline width cap

The navbar logo carried an inline `height:36px;width:auto;max-width:{logo_width}px`
while `.navbar-logo > img` already owns `height:36px;padding:0 30px`. Under
`box-sizing:border-box` that max-width capped the border box, so the 60px of
horizontal padding stayed inside it and wide logos were squeezed against a
fixed height, breaking the intrinsic aspect ratio.

- Render the nav logo with no inline sizing; CSS alone sizes it.
- Add a `$lazy` argument to jio_get_adaptive_theme_img() so the navbar can emit
  the real src up front instead of a square lazyload placeholder; defaults to
  'auto' so every other caller keeps the lazy_other behaviour.
- Remove the superseded logo_width plumbing: the setting (PHP + VUE schemas),
  Settings::logo_width(), jio_get_site_logo_width(), the --jio-logo-width token
  and the unused has_logo context key.
- Cover the contract in the header reference test.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

assets/admin/src/schema/sections/global/logo.js

@@ -13,7 +13,6 @@ export const logoSchema = {
     site_logo_dark: '',
     site_favicon: defaultFavicon,
     site_icon: defaultFavicon,
-    logo_width: 150,
     logo_text: '',
   },
   groups: [
@@ -24,7 +23,6 @@ export const logoSchema = {
       fields: [
         { key: 'site_logo', label: '站点 Logo', type: 'media', help: '建议尺寸 300x80，支持 PNG/SVG 透明背景' },
         { key: 'site_logo_dark', label: '暗色模式 Logo', type: 'media', help: '暗色模式下显示的 Logo，留空则使用默认 Logo' },
-        { key: 'logo_width', label: 'Logo 宽度 (px)', type: 'number', min: 50, max: 400, step: 10, help: '导航栏中 Logo 显示宽度' },
         { key: 'logo_text', label: 'Logo 文字', type: 'input', placeholder: '当无 Logo 图片时显示的文字', help: '同时作为首页标题和 alt 文本使用' },
       ],
     },