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/pages/Dashboard.tsx

@@ -4,7 +4,7 @@
 import { useState, useEffect, useCallback } from 'react';
 import { useAuth } from '../contexts/AuthContext';
 
-const API_BASE = 'http://localhost:3001';
+const API_BASE = import.meta.env.VITE_API_BASE || '';
 
 interface Project {
   id: string;