feat: replace backend/slot toggle with role selector

The backend toggle now cycles through configured roles (chat, coder,
research, distill, etc.) instead of backup model slots within the chat
role. Each role uses its own primary→backup chain from the registry.

- ChatRequest.slot replaced by chat_role (default "chat")
- GET /backend returns available_roles instead of chat_models
- _available_roles_for_toggle() builds list from defined_roles, excluding
  orchestrator (which has its own Agent mode)
- Model label on responses now reflects the actual role's assigned model
- Toggle is inert when only one role is configured (avoids useless cycling)
- Add "Clear browser cache" button to Account Settings (Connected Accounts)
- Add _role_model_label() helper for cleaner response tag labeling

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

cortex/static/settings.html

@@ -269,6 +269,24 @@
       </p>
     </div>
 
+    <!-- Browser cache -->
+    <div class="section">
+      <h2>Browser Cache</h2>
+      <p style="font-size:0.8rem; color:#94a3b8; margin-bottom:0.85rem; line-height:1.55;">
+        Clears UI preferences stored in this browser: active mode, session ID, memory toggles,
+        theme, font size, and context tier. Does not sign you out.
+      </p>
+      <button type="button" id="clear-ls-btn"
+              style="padding:0.5rem 1rem; background:none; border:1px solid #2d3148; border-radius:6px;
+                     color:#94a3b8; font-size:0.88rem; font-weight:500; cursor:pointer;
+                     transition:border-color 0.15s, color 0.15s;">
+        Clear browser cache
+      </button>
+      <span id="clear-ls-ok" style="display:none; margin-left:0.75rem; font-size:0.8rem; color:#4ade80;">
+        Cleared.
+      </span>
+    </div>
+
     <!-- Model Registry link -->
     <div class="section">
       <h2>Model Registry</h2>
@@ -349,6 +367,12 @@
       });
     }
 
+    // Clear localStorage (keeps JWT cookie — no sign-out)
+    document.getElementById('clear-ls-btn').addEventListener('click', () => {
+      localStorage.clear();
+      document.getElementById('clear-ls-ok').style.display = 'inline';
+    });
+
     // Persona rename toggle
     document.querySelectorAll('.persona-rename-toggle').forEach(btn => {
       btn.addEventListener('click', () => {