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/style.css

@@ -163,7 +163,7 @@
             background: var(--surface);
             border: 1px solid var(--border);
             border-radius: 8px;
-            box-shadow: 0 4px 16px rgba(0,0,0,0.4);
+            box-shadow: 0 8px 24px var(--shadow);
             z-index: 200;
             overflow: hidden;
         }
@@ -171,7 +171,9 @@
         .persona-dropdown.open { display: block; }
 
         .persona-dropdown a {
-            display: block;
+            display: flex;
+            align-items: center;
+            gap: 8px;
             padding: 0.55rem 0.85rem;
             color: var(--text);
             text-decoration: none;
@@ -183,6 +185,12 @@
 
         .persona-dropdown a.active { color: var(--accent); font-weight: 600; }
 
+        .persona-dropdown .pd-emoji {
+            font-size: 1.1rem;
+            line-height: 1;
+            flex-shrink: 0;
+        }
+
         .persona-dropdown .pd-divider {
             border-top: 1px solid var(--border);
             margin: 0.25rem 0;
@@ -1412,52 +1420,6 @@
             padding-left: 12px;
         }
 
-        /* ── Auth warning banner ─────────────────────────────────── */
-        #auth-banner {
-            display: none;
-            align-items: center;
-            gap: 10px;
-            padding: 8px 20px;
-            background: rgba(160, 100, 0, 0.18);
-            border-bottom: 1px solid rgba(200, 140, 20, 0.45);
-            font-size: 0.82rem;
-            color: #c9a84c;
-        }
-
-        #auth-banner.show { display: flex; }
-        #auth-banner.expired {
-            background: rgba(120, 20, 20, 0.25);
-            border-color: rgba(200, 60, 60, 0.45);
-            color: var(--error-text);
-        }
-
-        #auth-banner-text { flex: 1; display: flex; flex-direction: column; gap: 2px; }
-        #auth-banner-msg  { font-weight: 500; }
-        #auth-banner-hint {
-            font-size: 0.76rem;
-            opacity: 0.8;
-        }
-        #auth-banner-hint code {
-            font-family: 'Courier New', monospace;
-            background: rgba(0,0,0,0.2);
-            border-radius: 3px;
-            padding: 0 4px;
-        }
-
-        #auth-banner-close {
-            background: none;
-            border: 1px solid currentColor;
-            border-radius: 4px;
-            color: inherit;
-            font-size: 0.7rem;
-            padding: 2px 7px;
-            cursor: pointer;
-            opacity: 0.7;
-            flex-shrink: 0;
-        }
-
-        #auth-banner-close:hover { opacity: 1; }
-
         /* ── Toasts ──────────────────────────────────────────────── */
         #toast-container {
             position: fixed;