feat(FN-3008): notify when model settings fall back to defaults

Merges FN-3008 to add a "fallback-used" notification system: the engine now emits events when AI model fallbacks are triggered, dispatches notifications via ntfy/webhook providers, surfaces a session banner in the dashboard, and exposes a settings toggle to enable or disable these alerts.

Fusion-Task-Id: FN-3008

packages/core/src/__tests__/global-settings.test.ts

@@ -317,7 +317,16 @@ describe("GlobalSettingsStore", () => {
       // After clear, reading back gives the default value
       // (either undefined on disk with default applied, or default written directly)
       const settings = await store.getSettings();
-      expect(settings.ntfyEvents).toEqual(["in-review", "merged", "failed", "awaiting-approval", "awaiting-user-review", "planning-awaiting-input", "gridlock"]);
+      expect(settings.ntfyEvents).toEqual([
+        "in-review",
+        "merged",
+        "failed",
+        "awaiting-approval",
+        "awaiting-user-review",
+        "planning-awaiting-input",
+        "gridlock",
+        "fallback-used",
+      ]);
     });
 
     it("handles concurrent updates safely via locking", async () => {

packages/core/src/__tests__/notification-dispatcher.test.ts

@@ -153,6 +153,7 @@ describe("NotificationDispatcher", () => {
       "awaiting-user-review",
       "planning-awaiting-input",
       "gridlock",
+      "fallback-used",
     ]);
     expect(DEFAULT_GLOBAL_SETTINGS.notificationProviders).toEqual([]);
   });