feat(desktop): warn when main-model switch leaves auxiliary tasks pinned to another provider (#40286)

Switching the main model never touches auxiliary slot pins (they're
independent, sticky per-task overrides). A user who switches main away
from a now-unpaid provider keeps paying 402s on every background aux call
until they manually reset those pins — silently, with no UI signal.

- /api/model/set scope:'main' now returns stale_aux: slots still pinned
  to a provider different from the new main (additive field).
- Desktop Model Settings shows a switch-time notice after Apply AND a
  persistent banner when any loaded aux slot mismatches the main provider,
  both wired to the existing 'Reset all to main' action.
- Never auto-clears pins — a dedicated cheaper aux model is a legitimate
  config; surface-and-offer instead of nuking.
- Fixes a stale pre-existing assertion in the panel test (main model now
  renders via selectors, not a standalone label).

hermes_cli/web_server.py

@@ -2248,6 +2248,36 @@ async def set_model_assignment(body: ModelAssignment):
                     _log.debug("apply_nous_managed_defaults skipped", exc_info=True)
 
             save_config(cfg)
+
+            # Surface auxiliary slots still pinned to a *different* provider than
+            # the new main one. Switching the main model does NOT touch aux pins
+            # (they're independent, sticky per-task overrides — see
+            # auxiliary_client._resolve_auto). A user who switches main away from
+            # a now-unpaid provider (e.g. nous with $0 balance) keeps paying 402s
+            # on every background aux call until they reset those pins. We never
+            # auto-clear them — pinning aux to a cheaper/different model is a
+            # legitimate config — but we tell the caller so the UI can offer a
+            # "reset to main" nudge instead of silently burning credits.
+            new_provider = provider.strip().lower()
+            stale_aux: list[dict] = []
+            aux_cfg = cfg.get("auxiliary", {})
+            if isinstance(aux_cfg, dict):
+                for slot in _AUX_TASK_SLOTS:
+                    slot_cfg = aux_cfg.get(slot)
+                    if not isinstance(slot_cfg, dict):
+                        continue
+                    slot_provider = str(slot_cfg.get("provider", "") or "").strip()
+                    if (
+                        slot_provider
+                        and slot_provider.lower() not in {"auto", ""}
+                        and slot_provider.lower() != new_provider
+                    ):
+                        stale_aux.append({
+                            "task": slot,
+                            "provider": slot_provider,
+                            "model": str(slot_cfg.get("model", "") or ""),
+                        })
+
             return {
                 "ok": True,
                 "scope": "main",
@@ -2255,6 +2285,7 @@ async def set_model_assignment(body: ModelAssignment):
                 "model": model,
                 "base_url": model_cfg.get("base_url", ""),
                 "gateway_tools": gateway_tools,
+                "stale_aux": stale_aux,
             }
 
         # scope == "auxiliary"