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/player/now_playing_screen.dart

@@ -98,6 +98,8 @@ class _ProgressBar extends StatelessWidget {
                 Slider(
                   value: valueMs,
                   max: maxMs,
+                  semanticFormatterCallback: (v) =>
+                      _fmt(Duration(milliseconds: v.round())),
                   onChanged: hasDuration
                       ? (v) => handler.seek(Duration(milliseconds: v.round()))
                       : null,
@@ -146,6 +148,7 @@ class _Controls extends StatelessWidget {
           mainAxisAlignment: MainAxisAlignment.spaceEvenly,
           children: [
             IconButton(
+              tooltip: 'Zufallswiedergabe',
               icon: Icon(Icons.shuffle,
                   color: shuffle ? MeloTheme.red : Colors.white54),
               onPressed: () => handler.setShuffleMode(shuffle
@@ -153,22 +156,26 @@ class _Controls extends StatelessWidget {
                   : AudioServiceShuffleMode.all),
             ),
             IconButton(
+              tooltip: 'Vorheriger Titel',
               iconSize: 40,
               icon: const Icon(Icons.skip_previous),
               onPressed: handler.skipToPrevious,
             ),
             IconButton(
+              tooltip: playing ? 'Pause' : 'Abspielen',
               iconSize: 64,
               icon: Icon(playing ? Icons.pause_circle : Icons.play_circle,
                   color: MeloTheme.red),
               onPressed: playing ? handler.pause : handler.play,
             ),
             IconButton(
+              tooltip: 'Nächster Titel',
               iconSize: 40,
               icon: const Icon(Icons.skip_next),
               onPressed: handler.skipToNext,
             ),
             IconButton(
+              tooltip: 'Wiederholen',
               icon: Icon(
                 repeat == AudioServiceRepeatMode.one
                     ? Icons.repeat_one