Fix critical data loss bugs and improve error handling

## Critical Fixes
- Fix CRITICAL-1: Empty scans no longer delete entire library
  - Use timestamp-based tombstone logic instead of path-based
  - Only mark missing if livePaths is not empty

- Fix CRITICAL-2: Re-scanned files now return from deleted state
  - Add deleted=false to SongsCompanion.insert in both scanners

## High Priority Fixes
- HIGH-1: Fix SQLite variable limit crash on large libraries
  - Replace isNotIn() with timestamp comparison (O(1) not O(n))

- HIGH-2: Fix UI freezing during scan
  - Throttle progress updates (every 50 files or end of scan)

- HIGH-3: Add error handling for scan failures
  - Wrap scanFolders/scanAndroidMediaStore in try/catch
  - Display scanError banner in LibraryScreen

- HIGH-4: Add error handling for playback failures
  - Wrap loadPlaylist in try/catch

## Medium Priority Fixes
- MEDIUM-1: Replace ! with ?? to handle unknown ProcessingState
- MEDIUM-2: Escape LIKE wildcards in search queries
- MEDIUM-4: Display StreamBuilder errors instead of treating as empty
- MEDIUM-8: Add tooltips to all IconButtons for accessibility
- Add doc comments to public database APIs

## Testing
- Add regression tests for CRITICAL-1 and CRITICAL-2
- All 9 existing tests pass

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

lib/library/android_scan.dart

@@ -44,11 +44,15 @@ Future<int> scanAndroidMediaStore(
 
     var coverPath = prev?.coverPath;
     if (coverPath == null) {
-      final art = await audioQuery.queryArtwork(s.id, ArtworkType.AUDIO, size: 512);
-      if (art != null && art.isNotEmpty) {
-        final f = File(p.join(covers.path, '$id.img'));
-        await f.writeAsBytes(art);
-        coverPath = f.path;
+      try {
+        final art = await audioQuery.queryArtwork(s.id, ArtworkType.AUDIO, size: 512);
+        if (art != null && art.isNotEmpty) {
+          final f = File(p.join(covers.path, '$id.img'));
+          await f.writeAsBytes(art);
+          coverPath = f.path;
+        }
+      } catch (_) {
+        // Kaputtes Cover verzögert nicht den Scan.
       }
     }
 
@@ -63,11 +67,17 @@ Future<int> scanAndroidMediaStore(
       dateAddedMs: prev?.dateAddedMs ??
           (s.dateAdded != null ? s.dateAdded! * 1000 : now),
       updatedAtMs: now,
+      deleted: const Value(false),
     ));
-    onProgress?.call(++done, songs.length);
+    ++done;
+    if (done % 50 == 0 || done == songs.length) {
+      onProgress?.call(done, songs.length);
+    }
   }
 
   await db.upsertSongs(companions);
-  await db.markMissing(livePaths, now);
+  if (livePaths.isNotEmpty) {
+    await db.markMissing(now);
+  }
   return companions.length;
 }