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/ext.go

@@ -76,6 +76,12 @@ type Extension struct {
 	// This is currently being used for Android to use the Storage Access Framework.
 	fileOps FileOps
 
+	// directFileOps, when true, means that files received via fileOps should be
+	// delivered directly to the caller (DirectFileMode=true). Set by SetFileOps.
+	// SetStagedFileOps leaves this false so that received files are staged for
+	// explicit retrieval via WaitingFiles/OpenFile (used by the WASM JS bridge).
+	directFileOps bool
+
 	nodeBackendForTest ipnext.NodeBackend // if non-nil, pretend we're this node state for tests
 
 	mu             sync.Mutex // Lock order: lb.mu > e.mu
@@ -155,9 +161,10 @@ func (e *Extension) onChangeProfile(profile ipn.LoginProfileView, _ ipn.PrefsVie
 	// Use the provided [FileOps] implementation (typically for SAF access on Android),
 	// or create an [fsFileOps] instance rooted at fileRoot.
 	//
-	// A non-nil [FileOps] also implies that we are in DirectFileMode.
+	// A non-nil [FileOps] with directFileOps=true implies DirectFileMode (Android SAF).
+	// A non-nil [FileOps] with directFileOps=false uses staged mode (WASM JS bridge).
 	fops := e.fileOps
-	isDirectFileMode := fops != nil
+	isDirectFileMode := fops != nil && e.directFileOps
 	if fops == nil {
 		var fileRoot string
 		if fileRoot, isDirectFileMode = e.fileRoot(uid, activeLogin); fileRoot == "" {