feat(stream): report watch progress to API via HTTP Range tracking

Track the highest byte offset served by the stream server to estimate
playback progress (0-100%). A WatchReporter goroutine sends progress
to POST /api/internal/agent/watch-progress every 10s during streaming.

- Add maxByteOffset + totalFileSize to StreamServer for Range tracking
- Add FileSize() to fileProvider interface (all 3 providers)
- New WatchReporter: periodic progress reporter tied to daemon context
- New WatchProgressUpdate type with optional progress/position/duration
- Wire reporter into all 3 stream paths (task stream, disk stream, active download stream)

internal/cmd/stream_handler.go

@@ -55,7 +55,7 @@ func cancelStreamTask(taskID string) {
 // handleStreamTask manages a streaming task lifecycle outside the Manager.
 // It creates a StreamEngine, buffers, starts an HTTP server, and reports
 // progress until the task is cancelled or the download completes.
-func handleStreamTask(parentCtx context.Context, at agent.Task, reporter *engine.ProgressReporter, cfg config.Config) {
+func handleStreamTask(parentCtx context.Context, at agent.Task, reporter *engine.ProgressReporter, cfg config.Config, agentClient *agent.Client) {
 	ctx, cancel := context.WithCancel(parentCtx)
 	defer cancel()
 
@@ -121,6 +121,12 @@ func handleStreamTask(parentCtx context.Context, at agent.Task, reporter *engine
 	task.StreamURL = streamURL
 	log.Printf("[%s] stream ready: %s", at.ID[:8], streamURL)
 
+	// 5b. Start watch progress reporter (tracks Range requests for playback position)
+	if agentClient != nil {
+		watchReporter := engine.NewWatchReporter(agentClient, srv, at.ID)
+		go watchReporter.Run(ctx)
+	}
+
 	// 6. Unified progress + idle timeout loop
 	eng.StartProgressLoop(ctx)
 	progressTicker := time.NewTicker(3 * time.Second)