feat(M002): smart playlists — rule engine, editor UI, sidebar integration

Recovered from orphaned worktree commits (complete-milestone failed to merge).

Backend:
- Migration 9: is_smart + smart_rules columns on playlists table
- smartplaylist package: parameterized WHERE clause builder, field whitelist, genre subquery
- playlist.Service: Create/Update/Evaluate/Preview/GetRules smart playlist methods
- 65 tests (49 rule engine + 15 service + 1 migration)

Frontend:
- yj-combobox: reusable typeable dropdown with keyboard nav, ARIA, blur-race fix
- smart-playlist-editor: row-based rule builder with live preview
- smart-playlist-details: evaluate, refresh, play, shuffle, edit rules
- Sidebar: filter icon, Smart badge, create button, routing
- Queue snapshot on play/shuffle

backend/database/sql/schemas/playlists.sql

@@ -1,6 +1,8 @@
 CREATE TABLE IF NOT EXISTS playlists (
     id INTEGER PRIMARY KEY,
     name TEXT NOT NULL,
+    is_smart INTEGER NOT NULL DEFAULT 0,
+    smart_rules TEXT,
     created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     updated_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
 );