add tag-album and clean-playlist-files tasks, script step support, stop fix, rename fix

- New tag-album task: renames tracks to NNN format, sets ID3 tags via mutagen
- New clean-playlist-files task: deletes .cue, .m3u, .nfo and similar junk files
- Pipeline executor now supports script-mode steps (runs directly, no agent)
- Build discography pipeline: convert-audio → clean-playlist-files → prepare → fetch → tag
- Fix stop button: abort signal now kills running agent processes
- Fix job manager: broadcast stopped/error events to WebSocket viewers
- Fix file browser rename: delay focus to avoid context menu close race, left-align input

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

seed/tasks/build-discography/TASK.md

@@ -11,6 +11,14 @@ inputs:
     description: Artist / band name
     autofill: entry_name
 steps:
+  - task: convert-audio
+    inputs:
+      file_path: .
+      target_format: mp3
+      delete_source: "true"
+  - task: clean-playlist-files
+    inputs:
+      file_path: .
   - task: prepare-discography
     inputs:
       artist_name: ${artist_name}
@@ -21,4 +29,10 @@ steps:
     inputs:
       artist_name: ${artist_name}
       album_name: ${folder_name}
+  - task: tag-album
+    foreach: subdirectory
+    concurrency: 5
+    inputs:
+      artist_name: ${artist_name}
+      album_name: ${folder_name}
 ---