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/fileops_js.go

@@ -0,0 +1,16 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build js
+
+package taildrop
+
+import "errors"
+
+func init() {
+	// On WASM there is no real filesystem. newFileOps is only reached when
+	// SetFileOps was not called; return a clear error rather than panicking.
+	newFileOps = func(dir string) (FileOps, error) {
+		return nil, errors.New("taildrop: no filesystem on WASM; provide fileOps in the IPN config")
+	}
+}