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>

docs/MODULE_REGISTRY.md

@@ -135,12 +135,11 @@ Alle 76 Module der Mana-App (`apps/mana/apps/web/src/lib/modules/`).
 | `feedback` | Feedback | Bug-Reports und Feedback |
 | `playground` | Playground | Dev/Test-Playground |
 
-## Sonstige (5)
+## Sonstige (4)
 
 | Modul | Name | Beschreibung |
 |---|---|---|
 | `spiral` | Spiral | Spiral-Timeline-Visualisierung |
-| `arcade` | Arcade | AI-generierte Browser-Games |
 | `uload` | uLoad | URL-Shortener mit Click-Tracking |
 | `complexity` | Complexity | System-Komplexitäts-Metriken |
 | `core` | Core | Core-Infrastruktur und shared Exports |