Clean working version: deployed TypeScript backend + JSX frontend

- Replace old JS backend with working TypeScript backend from deployed containers
- Backend: Fastify + better-sqlite3 + Yjs CRDT, 17 TS source files, 13 test files
- Frontend: React JSX with fabric.js CAD canvas, vite build, nginx serving
- Fix nginx proxy port 5000→3001 to match backend
- Fix docker-compose port mapping 5000→3001
- Fix vite dev proxy port 5000→3001
- Add backend healthcheck to docker-compose
- Update index.html title to 'Web CAD', lang to 'de'
- Add .a0/ to .gitignore
- Remove old JS backend files (models, routes, config, middleware, seed)
- Remove tracked build artifacts (backend/public/)

backend/tests/health.test.ts

@@ -0,0 +1,45 @@
+/**
+ * Health Endpoint Tests
+ */
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import type { FastifyInstance } from 'fastify';
+import { SqliteAdapter } from '../src/database/SqliteAdapter.js';
+import { createServer } from '../src/server.js';
+
+describe('Health Endpoint', () => {
+  let app: FastifyInstance;
+  let db: SqliteAdapter;
+
+  beforeAll(async () => {
+    db = new SqliteAdapter(':memory:');
+    await db.init();
+    app = await createServer({ db, port: 0 });
+    await app.ready();
+  });
+
+  afterAll(async () => {
+    await app.close();
+    db.close();
+  });
+
+  it('GET /api/health should return ok status', async () => {
+    const response = await app.inject({
+      method: 'GET',
+      url: '/api/health',
+    });
+    expect(response.statusCode).toBe(200);
+    const body = JSON.parse(response.body);
+    expect(body.status).toBe('ok');
+    expect(body.timestamp).toBeDefined();
+  });
+
+  it('GET /api/health should return valid ISO timestamp', async () => {
+    const response = await app.inject({
+      method: 'GET',
+      url: '/api/health',
+    });
+    const body = JSON.parse(response.body);
+    const date = new Date(body.timestamp);
+    expect(date.toString()).not.toBe('Invalid Date');
+  });
+});