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,
     };
   }