feat(tsconnect): add notifyFilesWaiting and notifyIncomingFiles callbacks

Wire two new callbacks into the IPN notify stream:

- notifyFilesWaiting: fires when a completed inbound transfer is staged
  and ready to retrieve via waitingFiles(). Triggered by n.FilesWaiting
  in the notify stream.
- notifyIncomingFiles: fires with a JSON snapshot of in-progress inbound
  transfers whenever progress changes (roughly once per second while
  active, plus once at completion). The jsIncomingFile struct carries
  name, started (Unix ms), declaredSize, and received bytes. An empty
  array indicates all active transfers have finished.

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

cmd/tsconnect/wasm/wasm_js.go

@@ -394,6 +394,25 @@ func (i *jsIPN) run(jsCallbacks js.Value) {
 		if n.BrowseToURL != nil {
 			jsCallbacks.Call("notifyBrowseToURL", *n.BrowseToURL)
 		}
+		if n.FilesWaiting != nil {
+			jsCallbacks.Call("notifyFilesWaiting")
+		}
+		if n.IncomingFiles != nil {
+			files := make([]jsIncomingFile, len(n.IncomingFiles))
+			for i, f := range n.IncomingFiles {
+				files[i] = jsIncomingFile{
+					Name:         f.Name,
+					Started:      f.Started.UnixMilli(),
+					DeclaredSize: f.DeclaredSize,
+					Received:     f.Received,
+				}
+			}
+			if b, err := json.Marshal(files); err == nil {
+				jsCallbacks.Call("notifyIncomingFiles", string(b))
+			} else {
+				log.Printf("could not marshal IncomingFiles: %v", err)
+			}
+		}
 	})
 
 	go func() {
@@ -921,6 +940,15 @@ func (w termWriter) Write(p []byte) (n int, err error) {
 	return len(p), nil
 }
 
+// jsIncomingFile is the JSON representation of an in-progress inbound file
+// transfer sent to the notifyIncomingFiles callback.
+type jsIncomingFile struct {
+	Name         string `json:"name"`
+	Started      int64  `json:"started"`      // Unix milliseconds; use new Date(started) in JS
+	DeclaredSize int64  `json:"declaredSize"` // -1 if unknown
+	Received     int64  `json:"received"`     // bytes received so far
+}
+
 type jsNetMap struct {
 	Self      jsNetMapSelfNode   `json:"self"`
 	Peers     []jsNetMapPeerNode `json:"peers"`