feat: Phase 3 model toggle — cycle chat-role slot models in UI

Replaces the role-cycle toggle with a slot model toggle in the Context &
Memory panel. The active model label is shown on the button; clicking cycles
through Primary → Backup 1 → Backup 2 slots configured for the Chat role.

- app.js: remove activeRole()/availableRoles role-cycling; add
  activeChatModel()/chatModels slot cycling; update send/orchestrate
  payloads to send slot + chat_role:"chat"; fix updateSendBtnTitle and
  startRunTimer to use activeChatModel()
- chat.py: add slot field to ChatRequest; pass slot= to complete();
  resolve backend_label from slot config; add _chat_slot_models() helper;
  include chat_models in GET /backend response
- HELP.md: update Model toggle description, tool count (62/16),
  Backends section, API chat payload example

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

cortex/routers/chat.py

@@ -47,6 +47,7 @@ class ChatRequest(BaseModel):
     session_id: str | None = None
     tier: int | None = None
     model: str | None = None        # legacy backend override ("claude"|"gemini"|"local")
+    slot: str | None = None         # Phase 3: explicit slot ("primary"|"backup_1"|"backup_2")
     chat_role: str = "chat"         # active role: "chat"|"coder"|"research"|"distill" etc.
     include_long: bool = True
     include_mid: bool = True
@@ -109,6 +110,7 @@ async def _stream_chat(req: ChatRequest):
         messages=history,
         model=req.model,
         role=req.chat_role,
+        slot=req.slot,
     ))
 
     try:
@@ -124,7 +126,11 @@ async def _stream_chat(req: ChatRequest):
 
         try:
             response_text, actual_backend = task.result()
-            backend_label = _role_model_label(user, req.chat_role, actual_backend)
+            if req.slot:
+                slot_cfg = model_registry.get_model_for_slot(user, req.chat_role, req.slot)
+                backend_label = (slot_cfg or {}).get("label") or _role_model_label(user, req.chat_role, actual_backend)
+            else:
+                backend_label = _role_model_label(user, req.chat_role, actual_backend)
             host = platform.node()
             history.append({
                 "role": "assistant",
@@ -203,6 +209,25 @@ def _local_model_info(request: Request) -> dict | None:
     return None
 
 
+def _chat_slot_models(username: str) -> list[dict]:
+    """Return [{slot, label, type}] for each configured slot in the chat role, primary first."""
+    registry = model_registry.get_registry(username)
+    role_slots = registry.get("roles", {}).get("chat", {})
+    result = []
+    for slot_key in model_registry.PRIORITY_KEYS:
+        model_id = role_slots.get(slot_key)
+        if not model_id:
+            continue
+        resolved = model_registry._resolve_model(registry, model_id)
+        if resolved:
+            result.append({
+                "slot":  slot_key,
+                "label": resolved.get("label") or resolved.get("model_name") or "",
+                "type":  resolved.get("type", ""),
+            })
+    return result
+
+
 def _available_roles_for_toggle(username: str) -> list[dict]:
     """Return roles with a primary model assigned (excluding orchestrator) for the UI toggle.
 
@@ -231,6 +256,7 @@ def _available_roles_for_toggle(username: str) -> list[dict]:
 @router.get("/backend")
 async def get_backend(request: Request) -> dict:
     username        = _request_user(request)
+    chat_models     = _chat_slot_models(username) if username else []
     available_roles = _available_roles_for_toggle(username) if username else []
     p = settings.primary_backend
 
@@ -241,7 +267,8 @@ async def get_backend(request: Request) -> dict:
             orch_label = orch_cfg.get("label") or orch_cfg.get("model_name") or None
 
     return {
-        "available_roles":   available_roles,
+        "chat_models":        chat_models,       # Phase 3: [{slot, label, type}] for chat-role slots
+        "available_roles":    available_roles,    # kept for banner + backward compat
         "orchestrator_model": orch_label,
         # Legacy fields kept for backward compat
         "primary":     p,