refactor(app): move site routes into a (frontend) route group

Payload's admin panel ships its own root layout rendering html/body.
Next allows multiple root layouts only when no app/layout.tsx exists, so
the site's routes move into their own group. Route groups are invisible
to routing: every public URL is unchanged, verified against the build's
route table.

The metadata file conventions deliberately stay at the app/ root. Moving
them into the group renamed /icon.png to /icon-4usi79.png (likewise
apple-icon and opengraph-image) and dropped /robots.txt altogether,
which would have broken the /icon.png cache-control rule, the
outputFileTracingIncludes entry for the share card, and robots.txt.

darkThemeSafety reads app/globals.css off disk rather than importing it,
so it needed its own path fix — a grep for import specifiers misses it,
and it fails as an unrunnable suite rather than a failed assertion.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017YmbBhr8s7GusjDE12hrZM

nextjs-app/__tests__/api/proxyDenylist.test.ts

@@ -8,7 +8,7 @@
 // environment provides — under jsdom this suite fails on import, not on an
 // assertion.
 import { NextRequest } from 'next/server';
-import { GET } from '@/app/api/[...path]/route';
+import { GET } from '@/app/(frontend)/api/[...path]/route';
 
 function request(path: string) {
   return new NextRequest(`http://localhost:3000/api/${path}`);