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__/app/metadata.test.ts

@@ -1,7 +1,7 @@
-import { metadata as homeMetadata } from '@/app/page';
-import { metadata as rankingsMetadata } from '@/app/rankings/page';
-import { metadata as admissionsMetadata } from '@/app/admissions/page';
-import { generateMetadata as compareMetadata } from '@/app/compare/page';
+import { metadata as homeMetadata } from '@/app/(frontend)/page';
+import { metadata as rankingsMetadata } from '@/app/(frontend)/rankings/page';
+import { metadata as admissionsMetadata } from '@/app/(frontend)/admissions/page';
+import { generateMetadata as compareMetadata } from '@/app/(frontend)/compare/page';
 
 describe('canonical URLs', () => {
   it('the homepage canonicalises to the bare root', () => {

nextjs-app/__tests__/app/placeMetadata.test.ts

@@ -1,4 +1,4 @@
-import { generateMetadata as placeMeta } from '@/app/schools/[place]/page';
+import { generateMetadata as placeMeta } from '@/app/(frontend)/schools/[place]/page';
 
 jest.mock('@/lib/places', () => ({
   ...jest.requireActual('@/lib/places'),