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)
