feat(security): harden canvas server with auth, rate-limiting, and validation

- Add security.ts: helmet, CORS allowlist, timing-safe API key auth, prototype
  pollution guard, Mermaid input limits, rate limiting (general/destructive/burst)
- WS auth challenge-response with 5 s timeout and close code 4001
- Fix sync crash: array check before logger access (500 → 400)
- Fix sync/v2: validate element type before write (invalid → 400)
- Upgrade zod 3.22.4 → 3.25.5 (fixes ERR_PACKAGE_PATH_NOT_EXPORTED on startup)
- Extract ElementSharedFieldsSchema; move VALID_ELEMENT_TYPES to module level
- Docker: resource limits, .dockerignore hardening
- Add .project-hooks/pre-commit; expand test coverage (369 tests)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

playwright.config.ts

@@ -9,7 +9,7 @@ export default defineConfig({
   workers: 1,
   reporter: 'list',
   use: {
-    baseURL: 'http://localhost:3100',
+    baseURL: 'http://127.0.0.1:3100',
     trace: 'on-first-retry',
   },
   projects: [
@@ -25,8 +25,12 @@ export default defineConfig({
     reuseExistingServer: !process.env.CI,
     env: {
       CANVAS_PORT: '3100',
-      HOST: 'localhost',
+      HOST: '127.0.0.1',
       EXCALIDRAW_DB_PATH: '/tmp/excalidraw-e2e-test.db',
+      ALLOWED_ORIGINS: 'http://127.0.0.1:3100,http://localhost:3100,http://localhost:3000,http://127.0.0.1:3000',
+      EXCALIDRAW_RATE_LIMIT_GENERAL_MAX: '10000',
+      EXCALIDRAW_RATE_LIMIT_DESTRUCTIVE_MAX: '10000',
+      EXCALIDRAW_RATE_LIMIT_WRITE_BURST_MAX: '10000',
     },
   },
 });