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).

web/src/lib/api.ts

@@ -1608,6 +1608,14 @@ export interface ModelAssignmentRequest {
   task?: string;
 }
 
+/** An auxiliary task still pinned to a provider that differs from the
+ *  newly-selected main provider after a main-model switch. */
+export interface StaleAuxAssignment {
+  task: string;
+  provider: string;
+  model: string;
+}
+
 export interface ModelAssignmentResponse {
   ok: boolean;
   scope?: string;
@@ -1615,6 +1623,10 @@ export interface ModelAssignmentResponse {
   model?: string;
   tasks?: string[];
   reset?: boolean;
+  /** Auxiliary slots still pinned to a different provider than the new main.
+   *  Switching main never clears aux pins; this lets the UI warn the user
+   *  their helper tasks aren't following the switch. Only set on scope:'main'. */
+  stale_aux?: StaleAuxAssignment[];
 }
 
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