fix(tsconnect): guard nil n.Prefs in notify callback

n.Prefs is *PrefsView (a pointer), so calling n.Prefs.Valid() on a
Notify where Prefs is nil auto-dereferenced nil and panicked. The
callback's defer recover() swallowed the panic, which meant every
Notify without Prefs (Health-only, FilesWaiting, IncomingFiles,
OutgoingFiles, etc.) never reached the file-related JS calls.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

cmd/tsconnect/wasm/wasm_js.go

@@ -388,7 +388,7 @@ func (i *jsIPN) run(jsCallbacks js.Value) {
 				log.Printf("Could not generate JSON netmap: %v", err)
 			}
 		}
-		if n.Prefs.Valid() {
+		if n.Prefs != nil && n.Prefs.Valid() {
 			jsCallbacks.Call("notifyExitNode", string(n.Prefs.ExitNodeID()))
 		}
 		if n.BrowseToURL != nil {