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/hooks/useKeyboardShortcuts.ts

@@ -17,14 +17,8 @@ export function useKeyboardShortcuts(): void {
     function handleKeyDown(e: KeyboardEvent) {
       const ctrl = e.ctrlKey || e.metaKey;
 
-      // Ctrl+K: open command palette (always works, even in inputs)
-      if (ctrl && e.key === "k") {
-        e.preventDefault();
-        document.dispatchEvent(new CustomEvent("open-command-palette"));
-        return;
-      }
-
       // Skip remaining shortcuts when an input is focused
+      // Note: Ctrl+K for command palette is handled directly by CommandPalette component
       if (isInputFocused()) return;
 
       // Ctrl+Shift+Z: redo