fix(desktop): enable in-page drag-drop in Tauri build (#6548)

## Summary

- Set `dragDropEnabled: false` on the Tauri window so HTML5 drag events
reach the WebView. Previously the default `true` made Tauri intercept
all drag-drop at the OS level, silently breaking in-page drag-to-reorder
(Pragmatic Drag and Drop in `FileEditorThumbnail` /
`useFileItemDragDrop`) in the desktop build. The Active Files tab
reorder, which feeds Merge ordering, was the user-visible symptom.
- Browser builds are unaffected (tauri.conf.json is desktop-only).
- The OS file-drop pipeline now flows through the existing Mantine
`Dropzone` in `FileEditor.tsx` via HTML5 events instead of the Rust
`WindowEvent::DragDrop` handler in `lib.rs:215`. Verified working.

## Test plan

- [x] Desktop: drag a thumbnail in Active Files past another - row goes
semi-transparent, order updates on drop.
- [x] Desktop: drag a PDF from File Explorer onto the window - file is
added.
- [x] Web build: drag-to-reorder still works (unchanged code path; flag
is desktop-only).
- [x] Merge tool: order set by drag in Active Files is the order used by
the merge output.

## Follow-up (not in this PR)

- `WindowEvent::DragDrop` arm in
`frontend/editor/src-tauri/src/lib.rs:215-229` is now unreachable for
window drops. The `forward_files_to_window` helper still serves the
macOS Finder "Open With" path (`RunEvent::Opened` at lib.rs:230), so
only the DragDrop arm can be deleted. Worth a small cleanup pass later.

frontend/editor/src-tauri/tauri.conf.json

@@ -17,6 +17,7 @@
         "height": 800,
         "resizable": true,
         "fullscreen": false,
+        "dragDropEnabled": false,
         "additionalBrowserArgs": "--enable-features=CertVerifierBuiltinFeature"
       }
     ]