🧑‍💻 chore: add centralized environment variable system

- Add .env.development as single source of truth for dev variables
- Create scripts/generate-env.mjs to generate app-specific .env files
- Add pnpm setup:env command (also runs on postinstall)
- Update turbo.json with globalEnv for cache invalidation
- Add comprehensive docs/ENVIRONMENT_VARIABLES.md
- Update CLAUDE.md with env setup instructions

.gitignore

@@ -25,6 +25,9 @@ ios/
 .env.production.local
 .env*.local
 
+# BUT commit the central development env file
+!.env.development
+
 # IDE
 .idea/
 .vscode/