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/scan_service.dart

@@ -68,9 +68,13 @@ Future<int> scanFolders(
     var coverPath = prev?.coverPath;
     final pics = meta?.pictures ?? const <Picture>[];
     if (pics.isNotEmpty && coverPath == null) {
-      final f = File(p.join(covers.path, '$id.img'));
-      await f.writeAsBytes(pics.first.bytes);
-      coverPath = f.path;
+      try {
+        final f = File(p.join(covers.path, '$id.img'));
+        await f.writeAsBytes(pics.first.bytes);
+        coverPath = f.path;
+      } catch (_) {
+        // Kaputtes Cover verzögert nicht den Scan.
+      }
     }
 
     companions.add(SongsCompanion.insert(
@@ -83,11 +87,17 @@ Future<int> scanFolders(
       coverPath: Value(coverPath),
       dateAddedMs: prev?.dateAddedMs ?? now,
       updatedAtMs: now,
+      deleted: const Value(false),
     ));
-    onProgress?.call(++done, files.length);
+    ++done;
+    if (done % 50 == 0 || done == files.length) {
+      onProgress?.call(done, files.length);
+    }
   }
 
   await db.upsertSongs(companions);
-  await db.markMissing(livePaths, now);
+  if (livePaths.isNotEmpty) {
+    await db.markMissing(now);
+  }
   return companions.length;
 }