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/media-mark-192.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64" width="512" height="512"><defs><linearGradient id="bg" x1="0" y1="0" x2="1" y2="1"><stop offset="0" stop-color="#293542"/><stop offset="1" stop-color="#15191f"/></linearGradient><filter id="glow"><feGaussianBlur stdDeviation="2.2" result="b"/><feMerge><feMergeNode in="b"/><feMergeNode in="SourceGraphic"/></feMerge></filter></defs><rect x="5" y="5" width="54" height="54" rx="15" fill="url(#bg)" stroke="#60a9e6" stroke-width="2.5"/><path d="M17 21h30M17 31h19M17 41h30" fill="none" stroke="#f4c430" stroke-width="4" stroke-linecap="round"/><circle cx="46" cy="31" r="7" fill="#159447" stroke="#c9ffe0" stroke-width="2" filter="url(#glow)"/></svg>