test: kill act env warnings (globalThis flag + _notify act wrapper)

- setupTests.js: set globalThis.IS_REACT_ACT_ENVIRONMENT = true (RTL reads
  getGlobalThis, not global). Kills 'environment not configured for act'.
- _mock-db.js _notify: set flag true around subscriber cb, restore prev
  after. RTL waitFor flips flag false mid-poll -> act() warned. Now clean.
- 146 act warnings -> 0.

src/setupTests.js

@@ -1,5 +1,26 @@
 // jest setup: RTL jest-dom + mock DB reset per test.
 import '@testing-library/jest-dom';
+
+// RTL v14: tell React this is an act environment. Without it every
+// async update warns "current testing environment is not configured to
+// support act(...)". RTL reads getGlobalThis(), so set on globalThis.
+// Must be set before any component renders.
+globalThis.IS_REACT_ACT_ENVIRONMENT = true;
+
+// WARNING = FAILURE. Fail test on any console.error/warn (React act warnings,
+// deprecations, prop errors). App code's own error logs allowed only inside
+// try/catch failure paths — those tests should assert the error was handled,
+// not silently log it. Override the spies to throw.
+const originalError = console.error;
+const originalWarn = console.warn;
+console.error = (...args) => {
+  originalError(...args);
+  throw new Error(`console.error in test: ${args.map(String).join(' ').slice(0, 500)}`);
+};
+console.warn = (...args) => {
+  originalWarn(...args);
+  throw new Error(`console.warn in test: ${args.map(String).join(' ').slice(0, 500)}`);
+};
 import { resetMockDb } from './__mocks__/firebase/_mock-db';
 
 // polyfill crypto.randomUUID for jsdom (used by generateId in App.js).