Fix critical blocker issues and improve tests

## Critical Blockers Fixed
- Remove playbackState.addError() which broke audio_service permanently
  - Move error handling from audio_handler to song_list where UI context exists
  - Display error in SnackBar instead of destroying playback stream

- Add escapeChar parameter to LIKE queries for correct wildcard handling
  - Fixes broken search for titles with _ or % characters

- Add dismissScanError() method to allow closing error banner
  - Previously banner couldn't be dismissed (missing notifyListeners())

## Improvements
- Remove dead livePaths list, use companions.isNotEmpty instead
- Add initial progress update to show total files when scan starts
- Add tooltip to now_playing_screen Zurück button
- Add proper error handling in song_list with async/await

## Testing
- Add regression test for CRITICAL-1: empty scan doesn't delete songs
- Add regression test for CRITICAL-2: tombstoned files restore on re-scan
- All 11 tests pass

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

test/scan_test.dart

@@ -1,5 +1,6 @@
 import 'dart:io';
 
+import 'package:drift/drift.dart';
 import 'package:drift/native.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:melo/library/database.dart';
@@ -56,4 +57,46 @@ void main() {
     expect(secondIds, firstIds);
     expect((await db.watchSongs().first).length, 3); // keine Duplikate
   });
+
+  test('CRITICAL-1: leerer Scan loescht nicht (nonexistent folder)', () async {
+    // Initialer Scan
+    await scanFolders(db, [fixtures], coverDir: coverDir);
+    final initialCount = (await db.watchSongs().first).length;
+    expect(initialCount, 3);
+
+    // Scan mit nicht-existierendem Ordner (livePaths wird leer)
+    await scanFolders(db, ['/this/does/not/exist'], coverDir: coverDir);
+
+    // Alle Songs sollten noch da sein
+    final afterEmpty = await db.watchSongs().first;
+    expect(afterEmpty.length, initialCount);
+  });
+
+  test('CRITICAL-2: Datei wird nach Tombstone wieder sichtbar', () async {
+    // Initialer Scan: alle 3 Dateien
+    await scanFolders(db, [fixtures], coverDir: coverDir);
+    var songs = await db.watchSongs().first;
+    expect(songs.length, 3);
+    final nachtpulsId = songs.firstWhere((s) => s.title == 'Nachtpuls').id;
+
+    // Manuell tombstonen
+    await db.upsertSongs([
+      SongsCompanion.insert(
+        id: nachtpulsId,
+        path: p.join(fixtures, '01 Nachtpuls.mp3'),
+        title: 'Nachtpuls',
+        dateAddedMs: 1,
+        updatedAtMs: 1,
+        deleted: const Value(true),
+      ),
+    ]);
+    songs = await db.watchSongs().first;
+    expect(songs.where((s) => s.title == 'Nachtpuls').length, 0); // nicht sichtbar
+
+    // Re-Scan sollte die Datei wieder finden und deleted=false setzen
+    await scanFolders(db, [fixtures], coverDir: coverDir);
+    songs = await db.watchSongs().first;
+    expect(songs.where((s) => s.title == 'Nachtpuls').length, 1); // wieder da
+    expect(songs.length, 3); // alle 3 Songs
+  });
 }