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/App.css

@@ -69,6 +69,18 @@
   margin-top: 4px;
 }
 
+.optional-field label::after {
+  content: '';
+}
+
+.optional-field {
+  opacity: 0.7;
+}
+
+.optional-field:focus-within {
+  opacity: 1;
+}
+
 .create-project-btn {
   background-color: #0066ff;
   color: white;
@@ -86,7 +98,8 @@
   display: grid;
   grid-template-columns: 1fr 400px;
   gap: 20px;
-  min-height: 600px;
+  min-height: calc(100vh - 180px);
+  height: calc(100vh - 180px);
 }
 
 @media (max-width: 1200px) {
@@ -98,6 +111,8 @@
 .left-panel {
   display: flex;
   flex-direction: column;
+  height: 100%;
+  min-height: 0;
 }
 
 .right-panel {