Add utilities and initial test coverage

TESTING.md

@@ -0,0 +1,18 @@
+# Testing
+
+Run the unit and component test suite with:
+
+```
+npm test
+```
+
+For coverage reports:
+
+```
+npm run test:coverage
+```
+
+## Test utilities
+
+- `src/testUtils/mockDb.ts` provides a lightweight `createMockDb` factory with `MockTable` helpers that mimic Dexie tables used in the app.
+- `src/testUtils/stubDownloads.ts` stubs `document.createElement` and `URL.createObjectURL`/`revokeObjectURL` so download paths can be exercised in tests.