Add advanced editing features: layers, background removal, smart selection

Backend:
- Add /tools router with background removal, smart select, color select
- Add rembg dependency for AI background removal
- Add layer management API (list, flatten)
- Fix transparency preservation in blend_patch (veil collapse fix)
- Preserve alpha channel when reverting/resetting images

Frontend:
- Add AdvancedTools panel with background removal, smart select, color select
- Add Layers panel with drag-to-reorder, visibility toggle, flatten
- Add toolsApi for new backend endpoints
- Make right panel scrollable for additional controls

This adds "Photoshop light" capabilities:
- Remove background and create layer
- Smart object selection (click to select)
- Color selection with tolerance
- Layer system with compositing

frontend/src/components/AdvancedTools.css

@@ -0,0 +1,107 @@
+.advanced-tools {
+  background-color: #2a2a2a;
+  border-radius: 8px;
+  border: 1px solid #444;
+  padding: 16px;
+}
+
+.advanced-tools h3 {
+  margin: 0 0 16px 0;
+  font-size: 14px;
+  text-transform: uppercase;
+  letter-spacing: 0.5px;
+}
+
+.tool-group {
+  margin-bottom: 16px;
+  padding-bottom: 16px;
+  border-bottom: 1px solid #333;
+}
+
+.tool-group:last-of-type {
+  border-bottom: none;
+  margin-bottom: 0;
+  padding-bottom: 0;
+}
+
+.tool-group h4 {
+  font-size: 12px;
+  color: #888;
+  margin: 0 0 8px 0;
+  text-transform: uppercase;
+}
+
+.tool-buttons {
+  display: flex;
+  gap: 8px;
+  flex-wrap: wrap;
+}
+
+.tool-btn {
+  flex: 1;
+  min-width: 100px;
+  background-color: #3a3a3a;
+  color: #ffffff;
+  padding: 10px 12px;
+  font-size: 12px;
+  font-weight: 500;
+  border: 1px solid #555;
+  transition: all 0.2s;
+}
+
+.tool-btn:hover:not(:disabled) {
+  background-color: #4a4a4a;
+  border-color: #666;
+}
+
+.tool-btn.active {
+  background-color: #0066ff;
+  border-color: #0066ff;
+  color: white;
+}
+
+.tool-btn:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
+}
+
+.color-tolerance {
+  margin-top: 10px;
+  padding: 10px;
+  background-color: #333;
+  border-radius: 4px;
+}
+
+.color-tolerance label {
+  display: block;
+  font-size: 12px;
+  color: #888;
+  margin-bottom: 6px;
+}
+
+.color-tolerance input[type="range"] {
+  width: 100%;
+}
+
+.tool-hint {
+  font-size: 11px;
+  color: #00ff00;
+  margin-top: 8px;
+  padding: 8px;
+  background-color: rgba(0, 255, 0, 0.1);
+  border-radius: 4px;
+  text-align: center;
+}
+
+.cancel-mode-btn {
+  width: 100%;
+  margin-top: 12px;
+  background-color: #ff4444;
+  color: white;
+  padding: 8px;
+  font-size: 12px;
+}
+
+.cancel-mode-btn:hover {
+  background-color: #cc0000;
+}