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/canvas/ZoomPanController.ts

@@ -23,11 +23,13 @@ export class ZoomPanController {
   getViewport(): Viewport {
     const w = this.canvas.width / this.scale;
     const h = this.canvas.height / this.scale;
+    const minX = -this.offsetX / this.scale || 0;
+    const minY = -this.offsetY / this.scale || 0;
     return {
-      minX: -this.offsetX / this.scale,
-      minY: -this.offsetY / this.scale,
-      maxX: (-this.offsetX / this.scale) + w,
-      maxY: (-this.offsetY / this.scale) + h,
+      minX,
+      minY,
+      maxX: minX + w,
+      maxY: minY + h,
     };
   }
 

frontend/src/contexts/AuthContext.tsx

@@ -3,7 +3,7 @@
  */
 import { createContext, useContext, useState, useCallback, useEffect, type ReactNode } from 'react';
 
-const API_BASE = 'http://localhost:3001';
+const API_BASE = import.meta.env.VITE_API_BASE || '';
 
 export interface AuthUser {
   id: string;

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;

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 {