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/agent/client.go

@@ -178,6 +178,15 @@ func (c *Client) SyncLibrary(ctx context.Context, req LibrarySyncRequest) (*Libr
 	return &resp, nil
 }
 
+// ReportWatchProgress sends playback position to the server for watch tracking.
+func (c *Client) ReportWatchProgress(ctx context.Context, update WatchProgressUpdate) error {
+	var resp WatchProgressResponse
+	if err := c.doPost(ctx, "/api/internal/agent/watch-progress", update, &resp); err != nil {
+		return fmt.Errorf("watch progress: %w", err)
+	}
+	return nil
+}
+
 // doPost sends a JSON POST request and decodes the response.
 func (c *Client) doPost(ctx context.Context, path string, body any, dst any) error {
 	jsonBody, err := json.Marshal(body)

internal/agent/types.go

@@ -304,3 +304,24 @@ type LibrarySyncResponse struct {
 	Matched int `json:"matched"`
 	Removed int `json:"removed"`
 }
+
+// ---------------------------------------------------------------------------
+// Watch progress types (used by stream tracking)
+// ---------------------------------------------------------------------------
+
+// WatchProgressUpdate reports playback position during streaming.
+// Two modes:
+//   - Estimated (range): set Progress (0-100). Position/Duration omitted.
+//   - Precise (browser): set Position + Duration in seconds. Progress computed server-side.
+type WatchProgressUpdate struct {
+	TaskID   string `json:"taskId"`
+	Source   string `json:"source"`             // "range" or "browser"
+	Progress *int   `json:"progress,omitempty"` // 0-100 (range source)
+	Position *int   `json:"position,omitempty"` // seconds (browser source)
+	Duration *int   `json:"duration,omitempty"` // seconds (browser source)
+}
+
+// WatchProgressResponse is returned after reporting watch progress.
+type WatchProgressResponse struct {
+	Success bool `json:"success"`
+}