fix(pwa): serve apple-touch-icon + favicons from root, add sizes hint

iOS Safari's Add-to-Home-Screen flow probes /apple-touch-icon.png at the
site root in addition to the <link rel> on the page. Those root paths
currently 302 through Symfony's auth firewall to /login, so iOS gets HTML
where it expects a PNG and falls back to whatever it cached from earlier
installs (the 1 KB placeholder icon). Dropping the real PNG (and the
-precomposed alias) directly in public/ makes nginx serve them as static
files, ahead of the firewall.

Also adds favicon.svg and a multi-size favicon.ico at the root for
browsers/bots that probe / paths instead of reading <link>, and adds
sizes="180x180" to every apple-touch-icon link so iOS doesn't have to
guess.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/index.html

@@ -9,7 +9,7 @@
     <link rel="icon" type="image/png" sizes="32x32" href="/build/icons/favicon-32.png" />
     <link rel="icon" type="image/png" sizes="16x16" href="/build/icons/favicon-16.png" />
     <link rel="manifest" href="/build/manifest.webmanifest" />
-    <link rel="apple-touch-icon" href="/build/icons/apple-touch-icon.png" />
+    <link rel="apple-touch-icon" sizes="180x180" href="/build/icons/apple-touch-icon.png" />
 
     <meta name="theme-color" content="#fdf6ee" />
     <meta name="apple-mobile-web-app-capable" content="yes" />