fix(mobile): cold-start anchor for formSheet deep links

Without unstable_settings.anchor, a deep link or notification that
targets a formSheet route (issue/[id]/picker/status, etc.) cold-starts
the app onto the sheet alone — no parent screen, swipe-down lands the
user on a blank canvas. Anchor: '(tabs)' tells Expo Router to mount the
tab UI as the implicit base, so dismissing the sheet always returns to
a sensible workspace home.

Set on the workspace _layout.tsx that owns every formSheet route
registration. The root (app)/_layout has no formSheet declarations so
no anchor is needed there.

apps/mobile/app/(app)/[workspace]/_layout.tsx

@@ -44,6 +44,17 @@ const SHEET_OPTIONS: ComponentProps<typeof Stack.Screen>["options"] = {
   headerShown: false,
 };
 
+/**
+ * Cold-start deep-link anchor. Expo Router otherwise treats whatever
+ * route resolves the URL as the root of the stack — if the user opens a
+ * notification that targets `issue/[id]/picker/status` directly, they
+ * land on the formSheet with NO parent under it, no way to go back to
+ * the tabs. `anchor: "(tabs)"` tells the router to mount the tab UI as
+ * the implicit underlying screen so back/swipe-dismiss returns the user
+ * to a sensible base state.
+ */
+export const unstable_settings = { anchor: "(tabs)" } as const;
+
 /**
  * Mounts every per-feature realtime subscription. Lives inside
  * RealtimeProvider so the WSClient context is available, and stays alive