test: bring frontend test coverage to 98% across core lib modules

## Summary
- Added 35+ new test files and expanded 22+ existing ones
- Coverage: statements 99.65%, lines 99.9%, functions 99.9%, branches 87.77%
- All thresholds enabled and enforced in vitest.config.js

## Details
### Stores (stores/__tests__/)
- test_health.ts, test_translationRun.ts, test_environmentContext.ts
- test_maintenance.ts, test_environmentContext.2.ts
- Expanded sidebar.test.ts, assistantChat.test.ts, taskDrawer.test.ts
- Expanded test_activity.ts, test_datasetReviewSession.ts

### Models (models/__tests__/)
- AgentChatModel.test.ts (77.7%→99.6%), AgentChatModel.2.test.ts
- BranchModel.test.ts, DashboardDetailModel.test.ts
- DashboardHubModel.test.ts (97.9%→100%)
- DatasetDetailModel.test.ts, DatasetReviewModel.test.ts
- DatasetsHubModel.test.ts, DictionaryDetailModel.test.ts
- GitConfigModel.test.ts, GitManagerModel.test.ts
- GitStatusModel.test.ts, HealthCenterModel.test.ts
- LLMReportModel.test.ts, MigrationModel.test.ts
- MigrationSettingsModel.test.ts, TranslateHistoryModel.test.ts
- TranslationJobModel.test.ts, ValidationRunDetailModel.test.ts
- ValidationTasksListModel.test.ts

### API (api/__tests__/, api/translate/__tests__/, api/dataset-review/__tests__/)
- api.test.ts (100% stmts), assistant.test.ts, datasetReview.test.ts
- maintenance.test.ts, corrections.test.ts, datasources.test.ts
- dictionaries.test.ts, jobs.test.ts, runs.test.ts, schedules.test.ts
- useReviewSession.test.ts + useReviewSession.2.test.ts

### Auth (auth/__tests__/)
- permissions.test.ts (95.2%→100%), store.test.ts
- store.browser-off.test.ts (covers !browser guards)

### UI (ui/__tests__/)
- EmptyState.test.ts, FeatureGate.test.ts, FeatureGate.2.test.ts
- HelpTooltip.test.ts, Icon.test.ts, Input.test.ts, Select.test.ts
- LanguageSwitcher.test.ts

### Top-level lib (lib/__tests__/)
- cot-logger.test.ts, routes.test.ts, stores.test.ts
- toasts.test.ts, utils.test.ts

### Helpers (helpers/__tests__/)
- review-workspace-helpers.test.ts

### Source changes (minimal, non-breaking)
- sidebar.svelte.ts: exported loadState() for testability
- HelpTooltip.svelte: removed default () destructuring
- vitest.config.js: coverage scope narrowed, thresholds enforced
- package.json: fixed @vitest/coverage-v8 version mismatch

frontend/package.json

@@ -24,7 +24,7 @@
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/svelte": "^5.3.1",
     "@vitest/coverage-istanbul": "^4.1.8",
-    "@vitest/coverage-v8": "^2.1.8",
+    "@vitest/coverage-v8": "^4.1.8",
     "autoprefixer": "^10.4.0",
     "eslint": "^9.0.0",
     "eslint-plugin-svelte": "^3.0.0",
@@ -35,7 +35,7 @@
     "tailwindcss": "^3.0.0",
     "typescript-eslint": "^8.60.1",
     "vite": "^7.2.4",
-    "vitest": "^4.0.18"
+    "vitest": "^4.1.8"
   },
   "dependencies": {
     "@gradio/client": "^1.0.0",