fix: address code review findings — data loss, race condition, broken features

- TopBar: call closeBoard() before navigating back to prevent data loss
- board-store: guard debouncedSave against race condition when board is
  closed during an in-flight save
- board-store: add missing updatedAt to setColumnWidth
- useKeyboardShortcuts: remove duplicate Ctrl+K handler that prevented
  command palette from toggling closed
- AttachmentSection: wire up Tauri file dialog for adding attachments
  with link/copy mode support

src/stores/board-store.ts

@@ -63,6 +63,7 @@ function now(): string {
 
 function debouncedSave(
   board: Board,
+  get: () => BoardState & BoardActions,
   set: (partial: Partial<BoardState>) => void
 ): void {
   if (saveTimeout) clearTimeout(saveTimeout);
@@ -70,9 +71,14 @@ function debouncedSave(
     set({ saving: true });
     try {
       await saveBoard(board);
-      set({ saving: false, lastSaved: Date.now() });
+      // Only update state if the same board is still loaded
+      if (get().board?.id === board.id) {
+        set({ saving: false, lastSaved: Date.now() });
+      }
     } catch {
-      set({ saving: false });
+      if (get().board?.id === board.id) {
+        set({ saving: false });
+      }
     }
   }, 500);
 }
@@ -86,7 +92,7 @@ function mutate(
   if (!board) return;
   const updated = updater(board);
   set({ board: updated });
-  debouncedSave(updated, set);
+  debouncedSave(updated, get, set);
 }
 
 export const useBoardStore = create<BoardState & BoardActions>()(
@@ -166,6 +172,7 @@ export const useBoardStore = create<BoardState & BoardActions>()(
       setColumnWidth: (columnId, width) => {
         mutate(get, set, (b) => ({
           ...b,
+          updatedAt: now(),
           columns: b.columns.map((c) =>
             c.id === columnId ? { ...c, width } : c
           ),