test: comprehensive E2E and API test suite for full KanzlAI stack

Backend (Go):
- Expanded integration_test.go: health, auth middleware (expired/invalid/wrong-secret JWT),
  tenant CRUD, case CRUD (create/list/get/update/delete + filters + validation),
  deadline CRUD (create/list/update/complete/delete), appointment CRUD,
  dashboard (verifies all sections), deadline calculator (valid/invalid/unknown type),
  proceeding types & rules, document endpoints, AI extraction (no-key path),
  and full critical path E2E (auth -> case -> deadline -> appointment -> dashboard -> complete)
- New handler unit tests: case (10), appointment (11), dashboard (1), calculate (5),
  document (10), AI (4) — all testing validation, auth guards, and error paths without DB
- Total: ~80 backend tests (unit + integration)

Frontend (TypeScript/Vitest):
- Installed vitest 2.x, @testing-library/react, @testing-library/jest-dom, jsdom 24, msw
- vitest.config.ts with jsdom env, esbuild JSX automatic, path aliases
- API client tests (13): URL construction, no double /api/, auth header, tenant header,
  POST/PUT/PATCH/DELETE methods, error handling, 204 responses
- DeadlineTrafficLights tests (5): renders cards, correct counts, zero state, onFilter callback
- CaseOverviewGrid tests (4): renders categories, counts, header, zero state
- LoginPage tests (8): form rendering, mode toggle, password login, redirect, error display,
  magic link, registration link
- Total: 30 frontend tests

Makefile: test-frontend target now runs vitest instead of placeholder echo.

frontend/package.json

@@ -6,7 +6,9 @@
     "dev": "next dev --turbopack",
     "build": "next build --turbopack",
     "start": "next start",
-    "lint": "eslint"
+    "lint": "eslint",
+    "test": "vitest run",
+    "test:watch": "vitest"
   },
   "dependencies": {
     "@supabase/ssr": "^0.9.0",
@@ -21,14 +23,20 @@
     "sonner": "^2.0.7"
   },
   "devDependencies": {
-    "typescript": "^5",
+    "@eslint/eslintrc": "^3",
+    "@tailwindcss/postcss": "^4",
+    "@testing-library/jest-dom": "^6.9.1",
+    "@testing-library/react": "^16.3.2",
+    "@testing-library/user-event": "^14.6.1",
     "@types/node": "^20",
     "@types/react": "^19",
     "@types/react-dom": "^19",
-    "@tailwindcss/postcss": "^4",
-    "tailwindcss": "^4",
     "eslint": "^9",
     "eslint-config-next": "15.5.14",
-    "@eslint/eslintrc": "^3"
+    "jsdom": "24.1.3",
+    "msw": "^2.12.14",
+    "tailwindcss": "^4",
+    "typescript": "^5",
+    "vitest": "2.1.8"
   }
 }