fix: comment waveform integration with timestamps and avatars

- Add author_avatar_url to API schema and frontend interface
- Capture current playhead timestamp when creating comments
- Display user avatars in waveform markers instead of placeholders
- Improve marker visibility with better styling (size, borders, shadows)
- Fix TypeScript type errors for nullable timestamps
- Add debug logging for troubleshooting

This implements the full comment waveform integration as requested:
- Comments are created with exact playhead timestamps
- Waveform markers show at correct positions with user avatars
- Clicking markers scrolls to corresponding comments
- Backward compatible with existing comments without timestamps

api/src/rehearsalhub/schemas/comment.py

@@ -19,6 +19,7 @@ class SongCommentRead(BaseModel):
     body: str
     author_id: uuid.UUID
     author_name: str
+    author_avatar_url: str | None
     timestamp: float | None
     created_at: datetime
 
@@ -30,6 +31,7 @@ class SongCommentRead(BaseModel):
             body=getattr(c, "body"),
             author_id=getattr(c, "author_id"),
             author_name=getattr(getattr(c, "author"), "display_name"),
+            author_avatar_url=getattr(getattr(c, "author"), "avatar_url"),
             timestamp=getattr(c, "timestamp"),
             created_at=getattr(c, "created_at"),
         )