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.

package.json

@@ -8,7 +8,6 @@
   ],
   "dependencies": {
     "@testing-library/jest-dom": "^5.17.0",
-    "@testing-library/react": "^13.4.0",
     "@testing-library/user-event": "^13.5.0",
     "autoprefixer": "^10.4.19",
     "firebase": "^10.12.2",
@@ -47,5 +46,8 @@
       "last 1 firefox version",
       "last 1 safari version"
     ]
+  },
+  "devDependencies": {
+    "@testing-library/react": "^14.3.1"
   }
 }