fix(frontend): Phase 9 — restore index dashboard, fix login/CSRF, port legacy styles

- Index dashboard regains the 8 cards that were lost in the SPA port
  (3X-UI panel info, Operation Hours, System Load, Usage, Overall Speed,
  Total Data, IP Addresses, Connection Stats), plus a Config button that
  shows the live xray config.json. Version display falls back through
  panelUpdateInfo → window.__X_UI_CUR_VER__ → '?' so dev mode isn't blank.
- Xray config no longer hangs on load: useXraySetting surfaces failures
  instead of leaving a perpetual spinner, and the Vite dev proxy stops
  hijacking POST requests to migrated routes (only GETs get bypassed).
- Inbound page no longer throws __asyncLoader/emitsOptions errors —
  inbound.js was missing imports (NumberFormatter, SizeFormatter,
  Wireguard) and InboundList kept emitting after unmount.
- Login round-trip works after logout: a public /csrf-token endpoint
  bootstraps the SPA before authentication, axios caches the token
  module-level, and the dev 401 handler navigates to /login.html
  instead of reloading the dashboard into a redirect loop.
- legacy.css mirrors the legacy panel's surface/text variables so dark
  and ultra-dark themes match main; every SPA entry imports it.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/components/AppSidebar.vue

@@ -52,7 +52,7 @@ const tabs = computed(() => [
   { key: `${prefix}panel/inbounds`, icon: 'user',      title: t('menu.inbounds') },
   { key: `${prefix}panel/settings`, icon: 'setting',   title: t('menu.settings') },
   { key: `${prefix}panel/xray`,     icon: 'tool',      title: t('menu.xray') },
-  { key: `${prefix}logout/`,        icon: 'logout',    title: t('logout') },
+  { key: `${prefix}logout`,         icon: 'logout',    title: t('logout') },
 ]);
 
 const activeTab = ref([props.requestUri]);