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/clean-playlist-files/TASK.md

@@ -0,0 +1,18 @@
+---
+name: Clean Playlist Files
+description: Recursively delete .cue, .m3u, .m3u8, .pls, .wpl, .xspf and other playlist files.
+version: 1
+mode: script
+language: bash
+triggers:
+  - type: directory
+inputs:
+  file_path:
+    type: string
+    description: Path to a directory to clean.
+args: [file_path]
+---
+
+# Clean Playlist Files
+
+Remove playlist and cue sheet files from a directory tree.