feat(frontend): i18n expansion, admin/profile i18n, grid/list view fix, empty state

- Add 5 new locales (hi, ar, ru, ja, ko) — now 14 total
- Admin panel: 117 i18n keys, confirm modal, animated tabs, no inline handlers
- Profile page: 58 i18n keys with data-i18n attributes
- Fix i18n safeT() shadowing bug and translationsLoaded timing
- Fix grid/list view: list header no longer shows in grid mode on login
- Fix classList.toggle hidden sync for view switching across all nav functions
- Revert .hidden important that broke login page rendering
- Add files empty state (no_files + empty_hint) with translations
- Fix language selector dropdown scroll and styling
- Fix admin panel scroll with sticky tabs

static/login.html

@@ -30,8 +30,8 @@
                 <div class="auth-logo-text">OxiCloud</div>
             </div>
             
-            <h2 class="auth-title" id="language-title">Welcome to OxiCloud</h2>
-            <p class="language-subtitle" id="language-subtitle">Please select your language</p>
+            <h2 class="auth-title" id="language-title">Welcome!</h2>
+            <p class="language-subtitle" id="language-subtitle">Select your language to continue</p>
             
             <!-- Single selected language box with inline dropdown -->
             <div class="lang-picker" id="lang-picker">