feat: enhance PWA manifest with multi-size icons, window controls overlay, and German locale settings

Add id "/" and lang "de" fields to manifest.json. Add display_override with window-controls-overlay/standalone options and prefer_related_applications:false. Replace single media-mark.svg icon with 192x192 and 512x512 sized variants (media-mark-192.svg, media-mark-512.svg) containing gradient background, yellow strokes, and green circle with glow filter. Update service worker cache to v3 with scope "/" parameter

static/sw.js

@@ -1,4 +1,4 @@
-const CACHE_NAME = 'media-max-shell-v2';
+const CACHE_NAME = 'media-max-shell-v3';
 const SHELL = ['/static/media-mark.svg', '/static/manifest.json'];
 
 self.addEventListener('install', event => {