Wire Smart Select, Color Select, fix layer buttons, add zoom

Frontend changes:
- Wire Smart Select and Color Select to canvas click handlers
- Add externalSelection prop to ImageCanvas for displaying AI-generated selections
- Add zoom controls (mouse wheel + buttons) to ImageCanvas
- Fix layer buttons (New Layer, Delete, Duplicate) with proper handlers
- Lift advancedToolMode state to App.jsx for coordination between components
- Add tool mode indicator overlay on canvas

Backend changes:
- Update smart-select endpoint to return JSON with polygon and bbox data
- Update color-select endpoint to return JSON with polygon and bbox data
- Add _mask_to_polygon helper function using OpenCV contour detection
- Add cv2 and base64 imports to tools.py

API changes:
- smartSelect and colorSelect now return { polygon, bbox, mask_base64 }

frontend/src/components/ImageCanvas.css

@@ -43,3 +43,66 @@
   z-index: 10;
   white-space: nowrap;
 }
+
+/* Zoom controls */
+.zoom-controls {
+  position: absolute;
+  bottom: 10px;
+  right: 10px;
+  display: flex;
+  align-items: center;
+  gap: 4px;
+  background-color: rgba(0, 0, 0, 0.8);
+  padding: 6px 10px;
+  border-radius: 4px;
+  z-index: 10;
+}
+
+.zoom-controls button {
+  width: 28px;
+  height: 28px;
+  padding: 0;
+  font-size: 18px;
+  font-weight: bold;
+  background-color: #444;
+  color: white;
+  border: 1px solid #666;
+  border-radius: 4px;
+  cursor: pointer;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+}
+
+.zoom-controls button:hover {
+  background-color: #555;
+}
+
+.zoom-level {
+  color: white;
+  font-size: 12px;
+  min-width: 45px;
+  text-align: center;
+}
+
+/* Tool mode indicator */
+.tool-mode-indicator {
+  position: absolute;
+  top: 10px;
+  left: 50%;
+  transform: translateX(-50%);
+  background-color: rgba(0, 120, 255, 0.9);
+  color: white;
+  padding: 10px 20px;
+  border-radius: 4px;
+  font-size: 14px;
+  font-weight: 500;
+  z-index: 10;
+  white-space: nowrap;
+  animation: pulse 2s infinite;
+}
+
+@keyframes pulse {
+  0%, 100% { opacity: 1; }
+  50% { opacity: 0.7; }
+}