feat: last-used persona cookie, emoji dropdown, theme support, auth status move

- cx_last_persona cookie set on serve_ui; root/login/help/settings
  redirects use preferred persona from cookie instead of alphabetically first
- /api/personas returns [{name, emoji}] objects; persona switcher dropdown
  renders emoji + name with flex layout and .pd-emoji span
- Help, Settings, Model Registry pages apply localStorage theme on load
  (no flash); CSS variables for dark/light replacing all hardcoded hex values
- Claude CLI auth status moved from prominent chat banner to Anthropic
  provider block in Model Registry — live dot indicator (ok/warn/err)
- Auth banner removed from main chat UI (index.html, app.js, style.css)
- Add Model collapsed into Models section as <details> to shorten page
- Light-mode overrides for provider icons, model badges, ctx-badge, tags
  (Anthropic/Google/local colors now readable in both themes)
- Help page gains table, pre/code, hr styles for HELP.md rendered content

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/static/index.html

@@ -148,15 +148,6 @@
         </div>
     </div>
 
-    <!-- Auth warning banner — shown when Claude CLI token is near expiry -->
-    <div id="auth-banner">
-        <div id="auth-banner-text">
-            <span id="auth-banner-msg"></span>
-            <span id="auth-banner-hint"></span>
-        </div>
-        <button id="auth-banner-close" title="Dismiss">✕</button>
-    </div>
-
     <div id="messages"></div>
     <div id="session-id"></div>