feat(seekbar): configurable seekbar styles with animated preview

Add five seekbar styles selectable in Settings → Appearance:
Waveform, Line & Dot, Bar, Thick Bar, and Segmented. Each option
shows an animated canvas preview using requestAnimationFrame.

Style is persisted in authStore (seekbarStyle, default: waveform).
Translations added for all 7 languages.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/locales/de.ts

@@ -543,6 +543,13 @@ export const deTranslation = {
     infiniteQueue: 'Endlose Warteschlange',
     infiniteQueueDesc: 'Automatisch Zufallstitel anhängen wenn die Warteschlange leer wird',
     experimental: 'Experimentell',
+    seekbarStyle: 'Seekbar-Stil',
+    seekbarStyleDesc: 'Aussehen der Wiedergabe-Seekbar auswählen',
+    seekbarWaveform: 'Wellenform',
+    seekbarLinedot: 'Linie & Punkt',
+    seekbarBar: 'Balken',
+    seekbarThick: 'Dicker Balken',
+    seekbarSegmented: 'Segmentiert',
   },
   changelog: {
     modalTitle: 'Was ist neu',