feat(tsconnect): add outgoing file transfer progress notifications

- Export UpdateOutgoingFiles on taildrop.Extension so it can be called
  from outside the package (wasm bridge, package main).
- Wrap sendFile's PUT body with progresstracking.NewReader so bytes-sent
  is sampled roughly once per second during transfer.
- Create an OutgoingFile entry (with UUID, peer ID, name, declared size)
  before the PUT and call UpdateOutgoingFiles on each progress tick and
  on completion (setting Finished/Succeeded). This flows into the IPN
  notify stream as OutgoingFiles notifications.
- Add jsOutgoingFile struct and wire n.OutgoingFiles into a new
  notifyOutgoingFiles callback in run(), mirroring notifyIncomingFiles.

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

cmd/tsconnect/wasm/wasm_js.go

@@ -419,6 +419,26 @@ func (i *jsIPN) run(jsCallbacks js.Value) {
 				log.Printf("could not marshal IncomingFiles: %v", err)
 			}
 		}
+		if n.OutgoingFiles != nil {
+			files := make([]jsOutgoingFile, len(n.OutgoingFiles))
+			for i, f := range n.OutgoingFiles {
+				files[i] = jsOutgoingFile{
+					ID:           f.ID,
+					PeerID:       string(f.PeerID),
+					Name:         f.Name,
+					Started:      f.Started.UnixMilli(),
+					DeclaredSize: f.DeclaredSize,
+					Sent:         f.Sent,
+					Finished:     f.Finished,
+					Succeeded:    f.Succeeded,
+				}
+			}
+			if b, err := json.Marshal(files); err == nil {
+				jsCallbacks.Call("notifyOutgoingFiles", string(b))
+			} else {
+				log.Printf("could not marshal OutgoingFiles: %v", err)
+			}
+		}
 	})
 
 	go func() {
@@ -955,6 +975,19 @@ type jsIncomingFile struct {
 	Received     int64  `json:"received"`     // bytes received so far
 }
 
+// jsOutgoingFile is the JSON representation of an outgoing file transfer
+// sent to the notifyOutgoingFiles callback.
+type jsOutgoingFile struct {
+	ID           string `json:"id"`
+	PeerID       string `json:"peerID"`
+	Name         string `json:"name"`
+	Started      int64  `json:"started"`      // Unix milliseconds
+	DeclaredSize int64  `json:"declaredSize"` // -1 if unknown
+	Sent         int64  `json:"sent"`         // bytes sent so far
+	Finished     bool   `json:"finished"`
+	Succeeded    bool   `json:"succeeded"` // only meaningful when finished
+}
+
 type jsNetMap struct {
 	Self      jsNetMapSelfNode   `json:"self"`
 	Peers     []jsNetMapPeerNode `json:"peers"`