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/vitest.config.ts

@@ -0,0 +1,26 @@
+import { defineConfig } from "vitest/config";
+import path from "path";
+
+export default defineConfig({
+  test: {
+    environment: "jsdom",
+    setupFiles: ["./src/__tests__/setup.ts"],
+    include: ["src/**/*.test.{ts,tsx}"],
+    globals: true,
+    css: false,
+  },
+  resolve: {
+    alias: {
+      "@": path.resolve(__dirname, "./src"),
+    },
+  },
+  css: {
+    // Disable PostCSS processing — Tailwind v4's plugin isn't compatible with Vite's PostCSS loader
+    postcss: {
+      plugins: [],
+    },
+  },
+  esbuild: {
+    jsx: "automatic",
+  },
+});