chore: add .env.example template files

Add environment variable templates for:
- Root .env.example (all services)
- backend/.env.example (API configuration)
- frontend/.env.local.example (client config)

Copy to .env and fill in your secrets before running.

frontend/.env.local.example

@@ -1,10 +1,13 @@
-# API
+# Frontend Environment Variables
+# Copy to .env.local and fill in your values
+
+# API URL
 NEXT_PUBLIC_API_URL=http://localhost:3001
 
 # Solana
 NEXT_PUBLIC_SOLANA_NETWORK=devnet
 NEXT_PUBLIC_SOLANA_RPC_URL=https://api.devnet.solana.com
 
-# App
+# App Info
 NEXT_PUBLIC_APP_NAME=CoopCredits
 NEXT_PUBLIC_APP_URL=http://localhost:3000