feat: upload queue with queued status and cancel button

Multi-file drops now show queued/uploading distinction. Jobs start
as 'queued' and transition to 'uploading' when the HTTP request
begins. Queued items show a cancel button in the upload panel.
Cancelled jobs are skipped in the sequential upload loop.

frontend/src/canvas/image-drop.ts

@@ -242,9 +242,18 @@ export function setupDragDrop(
         continue;
       }
 
-      const placeholder = createPlaceholder(viewport, cursorX, cursorY);
       const jobId = uploads?.addJob(file, boardId);
 
+      // Check if job was cancelled while queued (user clicked cancel)
+      if (jobId && uploads?.isCancelled(jobId)) {
+        col++;
+        if (col >= cols) { col = 0; row++; cursorY += rowMaxH + GAP; rowMaxH = 0; }
+        continue;
+      }
+
+      const placeholder = createPlaceholder(viewport, cursorX, cursorY);
+      if (jobId) uploads?.startUpload(jobId);
+
       try {
         const res = await uploadImage(boardId, file, (p) => {
           if (jobId) uploads?.setProgress(jobId, p);