feat: progressive music streaming, library-first search, player fixes

- Stream downloads as they grow: raw best-audio (no mp3 transcode) served
  chunked from the stream endpoint mid-download, then with native Range
  once complete. Songs play within seconds of hitting Play.
- Fix searched songs never starting: reconcileMedia now wires the stream
  URL when a track that was preparing gains one; audio_url is stable for
  the whole lifecycle so the player never reloads mid-playback.
- Progress bars now parse yt-dlp stderr (stdout never carried [download]).
- Prefer downloaded songs: /media/api/search merges ready library matches
  ahead of YouTube results; search_music/play_song tooling and the agent
  prompt tell Hope to always pick in_library=true.
- Mute button turns solid red while the mic is muted.

agent/agent.py

@@ -213,10 +213,13 @@ SYSTEM_PROMPT = textwrap.dedent("""\
     - Ambiguous results → offer_song_choices with the top 2-3 options, name
       them briefly, and let the user pick by voice or in the player panel.
       Never start playback on a guess.
-    The library is permanent: list_library shows what is already downloaded,
-    and those tracks play instantly without searching. If the user names a
-    library track exactly, just play it. Control playback with pause_music,
-    resume_music, skip_song and seek_music; check what's on with now_playing.
+    Prefer what is already saved: search_music puts downloaded library
+    matches first (in_library=true) — always pick one of those over a fresh
+    YouTube result, and list_library shows the whole saved collection which
+    plays instantly. If the user names a library track exactly, just play it.
+    A brand-new song starts playing within seconds and finishes downloading
+    in the background. Control playback with pause_music, resume_music,
+    skip_song and seek_music; check what's on with now_playing.
     add_stream registers a live radio or direct audio URL as a library track.
     While music is playing you cannot hear the user — they will come back to
     you when they want to talk again, and the music pauses automatically.