fix(desktop): compile rfd after nil-NSOpenPanel guard (E0069)

[jmfcamp]

Summary

Follow-up to #20 (file-attach NSOpenPanel nil-guard). That patch vendored rfd 0.16.0 and added an early-out when +[NSOpenPanel openPanel] / +[NSSavePanel savePanel] is nil. The sync fallback in ModalFuture::new used a bare return;, but that function returns Self, so Apple Silicon just desktop-release-build failed with:

error[E0069]: `return;` in a function whose return type is not `()`
   --> vendor/rfd/src/backend/macos/modal_future.rs:113:21

The async sheet path’s return; is inside a () closure and was already valid. The sync path now clones the future state, completes it as NSModalResponseCancel (so callers get None / no path), and returns Self { state } — same cancel behavior as #20, just a type-correct constructor exit.

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. None found for the E0069 compile break.

Testing

• Linux CI source-string guards in macos_file_panel.rs now also include modal_future.rs and assert the constructor returns Self on the nil-panel path (catches a regression to return;).
• Rebuild on Apple Silicon with just desktop-release-build. rfd must compile (E0069 gone). Vite/chunk warnings and wry unused-unsafe / deprecated warnings are not the failure.
• Then attach a file on macOS 26. If the panel is nil, the picker must cancel / produce no path instead of aborting. fix(desktop): stop macOS attach from aborting on nil NSOpenPanel #20 intent is unchanged.