🔧 Fix: Add DATABASE_URL runtime support for category pages

- Updated Dockerfile to accept DATABASE_URL at runtime
- Added .env.example for frontend
- Created Coolify environment setup guide

Fixes category pages 404 error - DATABASE_URL needs to be set in Coolify

frontend/.env.example

@@ -0,0 +1,8 @@
+# Frontend Environment Variables
+# Copy to .env.local for local development
+
+# PostgreSQL Database Connection
+DATABASE_URL=postgres://burmddit:Burmddit2026@172.26.13.68:5432/burmddit
+
+# Node Environment
+NODE_ENV=production