feat(taildrop): fix DirectFileMode, void callbacks, and empty WaitingFiles

- Add SetStagedFileOps to Extension: sets fileOps without enabling
  DirectFileMode, so WASM clients use staged retrieval (WaitingFiles,
  OpenFile, DeleteFile) instead of direct-write mode.
- Add directFileOps bool field: SetFileOps (Android SAF) sets it true;
  SetStagedFileOps (WASM JS) leaves it false. onChangeProfile now uses
  `fops != nil && e.directFileOps` to determine DirectFileMode.
- Add jsCallVoid to jsFileOps: void ops (openWriter, write, closeWriter,
  remove) now use cb(err?: string) instead of cb(null, err: string).
- Fix waitingFiles() returning JSON null when no files are waiting:
  normalise nil slice to empty slice before marshalling.
- Update wireTaildropFileOps to call SetStagedFileOps.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

feature/taildrop/paths.go

@@ -18,10 +18,21 @@ func (e *Extension) SetDirectFileRoot(root string) {
 	e.directFileRoot = root
 }
 
-// SetFileOps sets the platform specific file operations. This is used
+// SetFileOps sets the platform-specific file operations. This is used
 // to call Android's Storage Access Framework APIs.
+// It implies DirectFileMode, so received files are delivered directly to the
+// caller rather than staged for retrieval via WaitingFiles/OpenFile.
 func (e *Extension) SetFileOps(fileOps FileOps) {
 	e.fileOps = fileOps
+	e.directFileOps = true
+}
+
+// SetStagedFileOps sets the platform-specific file operations without enabling
+// DirectFileMode. Received files are staged for explicit retrieval via
+// WaitingFiles, OpenFile, and DeleteFile. Used by the WASM JS bridge.
+func (e *Extension) SetStagedFileOps(fileOps FileOps) {
+	e.fileOps = fileOps
+	e.directFileOps = false
 }
 
 func (e *Extension) setPlatformDefaultDirectFileRoot() {