fix: replace hardcoded localhost:3001 with relative API URL + add canvas mock and integration test

- Fix frontend Network error: API_BASE now defaults to empty string (same-origin)
- Fix 3 files: api.ts, AuthContext.tsx, Dashboard.tsx
- Add Canvas 2D context mock in tests/setup.ts for jsdom
- Fix -0 vs +0 in ZoomPanController.getViewport()
- Add IntegrationWorkflow.test.ts (35 tests, full CAD workflow)

frontend/src/services/api.ts

@@ -3,7 +3,7 @@
  */
 import type { CADElement, CADLayer, BlockDefinition } from '../types/cad.types';
 
-const API_BASE = import.meta.env.VITE_API_BASE || 'http://localhost:3001';
+const API_BASE = import.meta.env.VITE_API_BASE || '';
 
 function authHeaders(token: string): Record<string, string> {
   return {