chore: extract arcade into standalone repo

Arcade lives as its own pnpm workspace at ~/Documents/Code/arcade
now, with no @mana/* coupling. This drops every reference and the
games/ directory from the monorepo.

Removes:
- games/ directory (89 files: web + server + 22 HTML games + screenshots)
- @arcade/web, @arcade/server pnpm workspace entries (games/* globs)
- arcade scripts in root package.json (4 scripts)
- arcade.mana.how from mana-auth trusted origins + CORS_ORIGINS
- arcade entries in mana-apps registry, app-icons, URL overrides
- arcade.mana.how from cloudflared tunnel + prometheus blackbox probes
- arcade-web service block in docker-compose.macmini.yml
- generate-env.mjs entries for arcade server + web
- BRANDING_ONLY 'arcade' entry in registry consistency spec
- dead arcade translation keys in GuestWelcomeModal (DE+EN)
- arcade mention in CLAUDE.md, authentication guideline, MODULE_REGISTRY

Verified:
- services/mana-auth/src/auth/sso-config.spec.ts: 8/8 pass
- pnpm install regenerates lockfile cleanly (-536 lines)
- no remaining 'arcade' refs outside historical snapshot docs

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

packages/shared-auth-ui/src/components/GuestWelcomeModal.svelte

@@ -64,7 +64,6 @@
 		inventory: ['Alles im Überblick behalten', 'Quelloffen & unabhängig', 'Privat by Design'],
 		uload: ['Links kürzen & verwalten', 'Quelloffen & unabhängig', 'Privat by Design'],
 		news: ['Nachrichten, kuratiert für dich', 'Quelloffen & unabhängig', 'Privat by Design'],
-		arcade: ['Spiele direkt im Browser', 'Quelloffen & unabhängig', 'Privat by Design'],
 		skilltree: ['Dein Fortschritt, sichtbar', 'Quelloffen & unabhängig', 'Privat by Design'],
 		food: ['Ernährung bewusst leben', 'Quelloffen & unabhängig', 'Privat by Design'],
 		wisekeep: ['Wissen bewahren & teilen', 'Quelloffen & unabhängig', 'Privat by Design'],
@@ -97,7 +96,6 @@
 		inventory: ['Keep track of everything', 'Open-source & independent', 'Private by design'],
 		uload: ['Shorten & manage links', 'Open-source & independent', 'Private by design'],
 		news: ['News, curated for you', 'Open-source & independent', 'Private by design'],
-		arcade: ['Games right in your browser', 'Open-source & independent', 'Private by design'],
 		skilltree: ['Your progress, visualized', 'Open-source & independent', 'Private by design'],
 		food: ['Mindful nutrition tracking', 'Open-source & independent', 'Private by design'],
 		wisekeep: ['Preserve & share knowledge', 'Open-source & independent', 'Private by design'],