fix(desktop): compile rfd after nil-panel dialog_callback move (E0382)

[jmfcamp]

Summary

Follow-up to #20 (vendor wry/rfd nil-NSOpenPanel guard) and #21 (bare return; → return Self on the sync path). Apple Silicon just desktop-release-build still failed in vendored rfd:

error[E0382]: borrow of moved value: `dialog_callback`
  --> vendor/rfd/src/backend/macos/modal_future.rs:98:21

dialog_callback is a non-Copy Fn. Stock rfd 0.16.0 builds the panel, then moves that callback into the sheet RcBlock once. #20 made build_modal return Option<D> and added a nil-panel cancel after that move, so the later dialog_callback(state, NSModalResponseCancel) was a use-after-move.

The async sheet path now completes as NSModalResponseCancel before creating the RcBlock — same cancel / no-path behavior, no second use of the moved Fn. Sync path from #21 is unchanged (return Self { state }).

Hula branding is unchanged (productName Hula Buzz, bundle id com.huladesk.buzz, hulabuzz://). No extra crates vendored; no Tauri/wry bump.

Related issue

N/A. Follow-up to #20 and #21. None found for the E0382 compile break.

Testing

• Source-string guards in macos_file_panel.rs now assert the async sheet path cancels the nil panel before RcBlock::new (catches a regression to use-after-move). Existing E0069 return Self { state } guard is unchanged. Those assertions pass.
• Isolated rustc repro of the capturing-Fn / RcBlock move: the old order is E0382; the new order compiles.
• This environment cannot run the Mac GUI or a full just desktop-tauri-test (no GTK). Do not treat CI as picker reproduction.

Verify on Apple Silicon / macOS 26:

1. Merge this PR.
2. Pull.
3. just desktop-release-build
4. Attach a file. rfd must compile (no E0382, no E0069). If the open/save panel is nil, the picker must cancel / produce no path instead of aborting.

Vite/chunk warnings and wry/rfd unused-unsafe warnings are not the failure.