feat: add Upload Manager with progress tracking and status panel

- UploadManager store: tracks jobs through uploading → processing → done/failed
- UploadPanel component: floating bottom-left widget showing upload progress,
  file names, sizes, status with auto-dismiss for completed items
- axios onUploadProgress wired for real-time upload percentage
- Video jobs transition to "processing" after upload, then "done" when
  media:job:update socket event arrives (reuses existing pipeline)
- Failed uploads show error message from server response
- Clear button to dismiss finished/failed items

frontend/src/pages/Editor.tsx

@@ -23,6 +23,8 @@ import VideoControls from '../components/VideoControls';
 import ShortcutsHelp from '../components/ShortcutsHelp';
 import MattermostImport from '../components/MattermostImport';
 import Minimap from '../components/Minimap';
+import UploadPanel from '../components/UploadPanel';
+import { UploadManager } from '../stores/uploadManager';
 import { InboxZone } from '../canvas/InboxZone';
 import { getItemWorldBounds } from '../canvas/SceneManager';
 import { VideoSprite } from '../canvas/sprites/VideoSprite';
@@ -60,6 +62,7 @@ export default function Editor({ isPublicView }: EditorProps) {
   const inboxZoneRef = useRef<InboxZone | null>(null);
   const clipboardRef = useRef<SceneItem[]>([]);
   const canvasContainerRef = useRef<HTMLDivElement>(null);
+  const [uploadManager] = useState(() => new UploadManager());
 
   // UI state
   const [activeTool, setActiveTool] = useState<ToolType>(ToolType.SELECT);
@@ -146,7 +149,7 @@ export default function Editor({ isPublicView }: EditorProps) {
   useCanvasSetup({
     boardData, resolvedBoardId, user, isPublicView,
     canvasRef, selectionRef, undoRef, syncRef, inboxZoneRef, canvasContainerRef,
-    onCanvasChange, showToast, setOnlineUsers, setSelectedLayerIds,
+    uploadManager, onCanvasChange, showToast, setOnlineUsers, setSelectedLayerIds,
   });
 
   // Tool activation
@@ -631,6 +634,9 @@ export default function Editor({ isPublicView }: EditorProps) {
           />
         )}
 
+        {/* Upload progress panel */}
+        <UploadPanel uploadManager={uploadManager} />
+
         {/* Toasts */}
         <div style={{
           position: 'absolute', bottom: '12px', left: '50%', transform: 'translateX(-50%)',