chore(brand): swap header logo to a real svg + ship favicons for portal-admin (#106)

## Summary

Two related brand-asset cleanups, bundled per request:

### portal-shell — header logo
- Replace [`apf-small.png`](apps/portal-shell/public/logos/) (a 7.6 KB raster we'd extracted from the original PNG-in-SVG and re-encoded through sharp) with [`apf-logo.svg`](apps/portal-shell/public/logos/apf-logo.svg) — an actual 1024×1024 vector export (2.1 KB). Sharper at any density, smaller payload, no rasterisation artefacts when zoomed.
- [`header.html`](apps/portal-shell/src/app/components/header/header.html) swaps `<img src=…>` accordingly.
- The wide-format `apf-portal.svg` stays in place for future surfaces (login splash, etc.).

### portal-admin — favicons + PWA manifest
Mirrors what PR #84 did for `portal-shell`:
- Copy the six favicon images (`favicon.ico`, `favicon.svg`, `favicon-96x96.png`, `apple-touch-icon.png`, `web-app-manifest-{192,512}.png`) into [`apps/portal-admin/public/favicons/`](apps/portal-admin/public/favicons/). Single visual identity across the two apps.
- Customise [`site.webmanifest`](apps/portal-admin/public/favicons/site.webmanifest) for admin: `name: "APF Portal Admin"`, `short_name: "Admin"`. Everything else (icons, theme-color, display) stays identical to portal-shell's manifest.
- Wire the `<link>` block + `<meta name="theme-color">` in [`apps/portal-admin/src/index.html`](apps/portal-admin/src/index.html).
- Remove the obsolete top-level `apps/portal-admin/public/favicon.ico` — now under `favicons/`, served via `<link rel="shortcut icon">`.

## Decision worth flagging

**Assets duplicated rather than shared via a lib.** Both apps ship their own copy of the 7 favicon files (~120 KB binary each). The alternative — a `shared-assets` (or extended `shared-tokens`) lib with the assets glob copied into each app's `dist/` at build time — is the architecturally tidier path, but introduces a build-config change with no real payoff at our scale. Revisit if a third surface (e.g., a future static landing page) ends up needing the same set.

## Verification

- `nx run-many -t lint test build --projects=portal-shell,portal-admin` — green.
- Both `dist/apps/{portal-shell,portal-admin}/browser/{en,fr}/favicons/` ship the seven expected files.
- Admin's emitted `site.webmanifest` carries `"name": "APF Portal Admin"`.
- Admin's emitted `index.html` carries the full `<link>` block + `theme-color` meta.
- Portal-shell ships `apf-logo.svg` in its `logos/` folder per locale; `apf-small.png` is gone.

## Test plan

- [x] Lint + test + build green.
- [x] Built outputs spot-checked (assets ship, manifest text correct, index.html wired).
- [ ] Manual: `nx serve portal-shell` → header shows the new SVG logo crisp at 1x / 2x / 3x DPR.
- [ ] Manual: `nx serve portal-admin` → tab favicon visible, dev tools → Application → Manifest shows "APF Portal Admin" with no errors.
- [ ] Manual: install portal-admin as a PWA from a Chromium browser → the install dialog reads "Install APF Portal Admin", home-screen icon uses the same family as portal-shell.

---------

Co-authored-by: Julien Gautier <julien.gautier@apf.asso.fr>
Reviewed-on: #106

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

@@ -0,0 +1,21 @@
+{
+  "name": "APF Portal Admin",
+  "short_name": "Admin",
+  "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"
+}

apps/portal-admin/src/index.html

@@ -5,7 +5,17 @@
     <title>APF Portal Admin</title>
     <base href="/" />
     <meta name="viewport" content="width=device-width, initial-scale=1" />
-    <link rel="icon" type="image/x-icon" href="favicon.ico" />
+
+    <!-- Favicons + PWA manifest. Same image assets as portal-shell —
+         single visual identity across the apps — with an admin-
+         specific webmanifest so the PWA install banner reads "APF
+         Portal Admin" / "Admin" instead of the portal-shell name. -->
+    <link rel="icon" type="image/svg+xml" href="favicons/favicon.svg" />
+    <link rel="icon" type="image/png" sizes="96x96" href="favicons/favicon-96x96.png" />
+    <link rel="shortcut icon" href="favicons/favicon.ico" />
+    <link rel="apple-touch-icon" sizes="180x180" href="favicons/apple-touch-icon.png" />
+    <link rel="manifest" href="favicons/site.webmanifest" />
+    <meta name="theme-color" content="#ffffff" />
   </head>
   <body>
     <app-root></app-root>