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>

cmd/tsconnect/wasm/wasm_js.go

@@ -160,6 +160,7 @@ func newIPN(jsConfig js.Value) map[string]any {
 	if err := ns.Start(lb); err != nil {
 		log.Fatalf("failed to start netstack: %v", err)
 	}
+	wireTaildropFileOps(lb, jsConfig.Get("fileOps"))
 	srv.SetLocalBackend(lb)
 
 	jsIPN := &jsIPN{
@@ -267,6 +268,33 @@ func newIPN(jsConfig js.Value) map[string]any {
 			}
 			return jsIPN.setExitNodeEnabled(args[0].Bool())
 		}),
+		"listFileTargets": js.FuncOf(func(this js.Value, args []js.Value) any {
+			return jsIPN.listFileTargets()
+		}),
+		"sendFile": js.FuncOf(func(this js.Value, args []js.Value) any {
+			if len(args) != 3 {
+				log.Printf("Usage: sendFile(stableNodeID, filename, data)")
+				return nil
+			}
+			return jsIPN.sendFile(args[0].String(), args[1].String(), args[2])
+		}),
+		"waitingFiles": js.FuncOf(func(this js.Value, args []js.Value) any {
+			return jsIPN.waitingFiles()
+		}),
+		"openWaitingFile": js.FuncOf(func(this js.Value, args []js.Value) any {
+			if len(args) != 1 {
+				log.Printf("Usage: openWaitingFile(name)")
+				return nil
+			}
+			return jsIPN.openWaitingFile(args[0].String())
+		}),
+		"deleteWaitingFile": js.FuncOf(func(this js.Value, args []js.Value) any {
+			if len(args) != 1 {
+				log.Printf("Usage: deleteWaitingFile(name)")
+				return nil
+			}
+			return jsIPN.deleteWaitingFile(args[0].String())
+		}),
 	}
 }