Add PWA manifest, service worker, and install hint.

Enables home-screen installation with per-viewer scope and platform-specific guidance in EN/DE.

Co-authored-by: Cursor <cursoragent@cursor.com>

templates/_pwa_head.html

@@ -0,0 +1,6 @@
+<link rel="manifest" href="{{ manifest_href }}">
+<meta name="theme-color" content="#1a1d27">
+<meta name="apple-mobile-web-app-capable" content="yes">
+<meta name="apple-mobile-web-app-status-bar-style" content="black-translucent">
+<link rel="apple-touch-icon" href="/static/icon-192.png">
+<script src="/static/pwa.js" defer></script>