Add comprehensive test suite: 105 unit + 167 E2E tests

- Vitest unit tests: smart paste parsing (37), cost calculations (21),
  oil translation (16), dialog system (12), with production data fixtures
- Cypress E2E tests: API CRUD (27), auth flow (8), recipe detail (10),
  search (12), oil reference (4), favorites (6), inventory (6),
  recipe management (10), diary (11), bug tracker (8), user management (13),
  cost parity (6), data integrity (8), responsive (9), performance (6),
  navigation (8), admin flow (5)
- Test coverage doc with prioritized gap analysis
- Found backend bug: POST /api/bug-reports/{id}/comment deletes the bug

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/vite.config.js

@@ -11,5 +11,9 @@ export default defineConfig({
   },
   build: {
     outDir: 'dist'
+  },
+  test: {
+    environment: 'jsdom',
+    globals: true,
   }
 })