Add core editing UX improvements

- Make canvas use full viewport height for larger image display
- Add download button to export edited images
- Implement Undo/Redo with Ctrl+Z / Ctrl+Y keyboard shortcuts
- Add transform controls for selections (move, resize, rotate)
- Add warn on reload to prevent data loss
- Make project naming optional, auto-generate from filename

frontend/src/components/ImageCanvas.css

@@ -2,11 +2,14 @@
   position: relative;
   width: 100%;
   height: 100%;
-  min-height: 500px;
+  min-height: calc(100vh - 180px);
   background-color: #2a2a2a;
   border: 2px solid #444;
   border-radius: 8px;
   overflow: hidden;
+  display: flex;
+  align-items: center;
+  justify-content: center;
 }
 
 .canvas-container canvas {
@@ -26,3 +29,17 @@
 .clear-selection-btn:hover {
   background-color: #cc0000;
 }
+
+.selection-hint {
+  position: absolute;
+  bottom: 10px;
+  left: 50%;
+  transform: translateX(-50%);
+  background-color: rgba(0, 0, 0, 0.8);
+  color: #00ff00;
+  padding: 8px 16px;
+  border-radius: 4px;
+  font-size: 12px;
+  z-index: 10;
+  white-space: nowrap;
+}