refactor: move supabase config to repo root, fix port and env setup

- Move supabase/ from packages/db/ to repo root for simpler CLI usage
- Use custom ports (553xx) to avoid conflicts with other local Supabase instances
- Add --env-mode=loose to turbo dev so root .env vars reach Next.js
- Remove hardcoded --port 3000 from next dev (reads PORT env var instead)
- Add .env.example with Supabase env var placeholders
- Add supabase local state dirs to .gitignore

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/editor/package.json

@@ -4,7 +4,7 @@
   "type": "module",
   "private": true,
   "scripts": {
-    "dev": "next dev --port 3000",
+    "dev": "next dev",
     "build": "next build",
     "start": "next start",
     "lint": "biome lint",