feat(portal-shell): full favicon set + PWA manifest

Drop in the realfavicongenerator-style asset bundle under
`public/favicons/`: SVG primary favicon, PNG fallback (96×96), legacy
ICO, Apple touch icon (180×180), and a Web App Manifest with 192/512
maskable PNGs for Android adaptive icons. Wire them in index.html via
the standard `<link rel="icon|apple-touch-icon|manifest">` block and
add a matching `<meta name="theme-color">` so the mobile chrome tints
align with the manifest.

PWA install banner reads "APF Portal" (short_name "Portal") on
home-screen and install prompts. theme + background colors stay white
to match the app header.

The old root-level `favicon.ico` is replaced by the one under
`public/favicons/` (referenced via `<link rel="shortcut icon">`).

apps/portal-shell/public/favicons/site.webmanifest

@@ -0,0 +1,21 @@
+{
+  "name": "APF Portal",
+  "short_name": "Portal",
+  "icons": [
+    {
+      "src": "/favicons/web-app-manifest-192x192.png?v=20260511",
+      "sizes": "192x192",
+      "type": "image/png",
+      "purpose": "maskable"
+    },
+    {
+      "src": "/favicons/web-app-manifest-512x512.png?v=20260511",
+      "sizes": "512x512",
+      "type": "image/png",
+      "purpose": "maskable"
+    }
+  ],
+  "theme_color": "#ffffff",
+  "background_color": "#ffffff",
+  "display": "standalone"
+}