feat(playlists): Songs per Bottom-Sheet zu Playlisten hinzufügen

Ohne Eintrittspunkt wären Playlisten unbenutzbar, da sie sonst nie Songs
erhalten könnten. SongList bekommt einen zusätzlichen Trailing-Button
(Icons.playlist_add) neben dem Favoriten-Herz, der ein Bottom-Sheet mit
allen Playlisten öffnet ("Neue Playlist" oben, dann bestehende). Antippen
hängt den Song ans Ende der gewählten Playlist an und zeigt eine SnackBar.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

lib/library/song_list.dart

@@ -2,9 +2,11 @@ import 'package:flutter/material.dart';
 import 'package:provider/provider.dart';
 
 import '../player/audio_handler.dart';
+import '../playlists/create_playlist_dialog.dart';
 import '../shared/cover.dart';
 import '../shared/favorite_button.dart';
 import 'database.dart';
+import 'playlist_service.dart';
 import 'song_media.dart';
 
 /// Scrollbare Songliste; Tippen spielt die ganze Liste ab dem Song ab.
@@ -12,6 +14,63 @@ class SongList extends StatelessWidget {
   const SongList(this.songs, {super.key});
   final List<Song> songs;
 
+  /// Öffnet ein Bottom-Sheet mit allen Playlisten, um [song] hinzuzufügen.
+  Future<void> _showAddToPlaylist(BuildContext context, Song song) async {
+    final db = context.read<MeloDb>();
+    final service = context.read<PlaylistService>();
+    await showModalBottomSheet(
+      context: context,
+      builder: (sheetContext) => SafeArea(
+        child: StreamBuilder<List<Playlist>>(
+          stream: db.watchPlaylists(),
+          builder: (context, snapshot) {
+            final playlists = snapshot.data ?? const [];
+            return Column(
+              mainAxisSize: MainAxisSize.min,
+              children: [
+                ListTile(
+                  leading: const Icon(Icons.add),
+                  title: const Text('Neue Playlist'),
+                  onTap: () async {
+                    final newId = await showDialog<String?>(
+                      context: sheetContext,
+                      builder: (_) => const CreatePlaylistDialog(),
+                    );
+                    if (newId == null) return;
+                    final position = (await db.watchPlaylistSongs(newId).first).length;
+                    await service.addSongToPlaylist(newId, song.id, position);
+                    if (sheetContext.mounted) {
+                      Navigator.pop(sheetContext);
+                      ScaffoldMessenger.of(sheetContext).showSnackBar(
+                        const SnackBar(content: Text('Zu Playlist hinzugefügt')),
+                      );
+                    }
+                  },
+                ),
+                for (final playlist in playlists)
+                  ListTile(
+                    leading: const Icon(Icons.playlist_play),
+                    title: Text(playlist.name),
+                    onTap: () async {
+                      final position =
+                          (await db.watchPlaylistSongs(playlist.id).first).length;
+                      await service.addSongToPlaylist(playlist.id, song.id, position);
+                      if (sheetContext.mounted) {
+                        Navigator.pop(sheetContext);
+                        ScaffoldMessenger.of(sheetContext).showSnackBar(
+                          SnackBar(content: Text('Zu ${playlist.name} hinzugefügt')),
+                        );
+                      }
+                    },
+                  ),
+              ],
+            );
+          },
+        ),
+      ),
+    );
+  }
+
   @override
   Widget build(BuildContext context) {
     final handler = context.read<MeloAudioHandler>();
@@ -28,7 +87,17 @@ class SongList extends StatelessWidget {
           title: Text(s.title, maxLines: 1, overflow: TextOverflow.ellipsis),
           subtitle: Text(s.artist ?? 'Unbekannt',
               maxLines: 1, overflow: TextOverflow.ellipsis),
-          trailing: FavoriteButton(songId: s.id),
+          trailing: Row(
+            mainAxisSize: MainAxisSize.min,
+            children: [
+              FavoriteButton(songId: s.id),
+              IconButton(
+                tooltip: 'Zu Playlist hinzufügen',
+                icon: const Icon(Icons.playlist_add),
+                onPressed: () => _showAddToPlaylist(context, s),
+              ),
+            ],
+          ),
           onTap: () async {
             try {
               await playSongs(handler, songs, i);