test: upgrade @testing-library/react v14, kill act warnings

@testing-library/react v13 -> v14. v13 used deprecated
ReactDOMTestUtils.act on every render -> 708 deprecation warnings.
v14 imports act from react directly. Drops to 0.

Mock _notify (src/__mocks__/firebase/_mock-db.js): wrap subscriber
callbacks in act(() => cb()). Sync setState from subscribe callbacks
fired outside test act boundary -> 14 warnings. Now 0. try/catch
fallback if react/act unavailable (defensive).

Full suite: 166 pass / 1 fail (Combat.scenario BUG-11 pre-existing
crash). Warnings: 1416 -> 0.

Note: NOT complete kill-shared. App.js still inlines 8 logic fns
(startEncounter, nextTurn, togglePause, endEncounter, addParticipant,
updateParticipant, reorder/drag). turn.js versions dead in app, used
by replay + turn tests only. Next: make handlers call turn.js.

src/__mocks__/firebase/_mock-db.js

@@ -38,11 +38,25 @@ export const MOCK_DB = {
     return () => state.subscribers.get(path)?.delete(cb);
   },
   _notify(path) {
-    // notify exact doc path subscribers
-    if (state.subscribers.has(path)) state.subscribers.get(path).forEach(cb => cb());
+    // notify exact doc path subscribers (wrapped in act for test isolation)
+    if (state.subscribers.has(path)) state.subscribers.get(path).forEach(cb => {
+      try {
+        const { act } = require('react');
+        act(() => cb());
+      } catch {
+        cb();
+      }
+    });
     // notify parent collection subscribers
     const parent = path.split('/').slice(0, -1).join('/');
-    if (parent && state.subscribers.has(parent)) state.subscribers.get(parent).forEach(cb => cb());
+    if (parent && state.subscribers.has(parent)) state.subscribers.get(parent).forEach(cb => {
+      try {
+        const { act } = require('react');
+        act(() => cb());
+      } catch {
+        cb();
+      }
+    });
   },
   nextId() { state.counter += 1; return String(state.counter).padStart(3, '0'); },
   _state: state,