feat(offline): add pending playlist sync queue

Backend:
- Add PendingPlaylist struct for offline-created playlists
- Create pending_playlist_sync table in SQLite
- Add methods: create_pending_playlist, get_pending_playlists,
  mark_playlist_synced, delete_pending_playlist, get_pending_playlist_count
- Register Tauri commands for pending playlist management

Assets:
- Add offline.svg icon for offline mode indicator

cc

static/offline.svg

@@ -0,0 +1,2 @@
+<?xml version="1.0" encoding="utf-8"?><!-- Uploaded to: SVG Repo, www.svgrepo.com, Generator: SVG Repo Mixer Tools -->
+<svg width="800px" height="800px" viewBox="0 0 512 512" xmlns="http://www.w3.org/2000/svg"><title>ionicons-v5-s</title><path d="M93.72,183.25C49.49,198.05,16,233.1,16,288c0,66,54,112,120,112H320.37" style="fill:none;stroke:#000000;stroke-linecap:round;stroke-linejoin:round;stroke-width:32px"/><path d="M467.82,377.74C485.24,363.3,496,341.61,496,312c0-59.82-53-85.76-96-88-8.89-89.54-71-144-144-144-26.16,0-48.79,6.93-67.6,18.14" style="fill:none;stroke:#000000;stroke-linecap:round;stroke-linejoin:round;stroke-width:32px"/><line x1="448" y1="448" x2="64" y2="64" style="fill:none;stroke:#000000;stroke-linecap:round;stroke-miterlimit:10;stroke-width:32px"/></svg>