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>

feature/taildrop/ext.go

@@ -418,6 +418,13 @@ func (e *Extension) taildropTargetStatus(p tailcfg.NodeView, nb ipnext.NodeBacke
 	return ipnstate.TaildropTargetAvailable
 }
 
+// UpdateOutgoingFiles updates the tracked set of outgoing file transfers and
+// sends an ipn.Notify with the full merged list. The updates map is keyed by
+// OutgoingFile.ID; existing entries not present in updates are preserved.
+func (e *Extension) UpdateOutgoingFiles(updates map[string]*ipn.OutgoingFile) {
+	e.updateOutgoingFiles(updates)
+}
+
 // updateOutgoingFiles updates b.outgoingFiles to reflect the given updates and
 // sends an ipn.Notify with the full list of outgoingFiles.
 func (e *Extension) updateOutgoingFiles(updates map[string]*ipn.OutgoingFile) {