feat: implement runtime configuration with API proxy pattern

Replace build-time NEXT_PUBLIC_* environment variables with server-side
runtime configuration. This allows changing the Pastel API URL without
rebuilding the Docker image.

**Changes:**
- Add Next.js API proxy route at /api/pastel/[...path] for server-side proxying
- Update API client to use proxy endpoint instead of direct API URL
- Replace NEXT_PUBLIC_API_URL with server-side PASTEL_API_URL
- Remove build arguments from Dockerfile (no longer needed)
- Simplify docker-compose.yml to use runtime environment variables only
- Update all .env files to reflect new configuration approach
- Add comprehensive DOCKER.md documentation

**Benefits:**
- No rebuild required to change API URL
- Same image works across all environments (dev/staging/prod)
- Better security (API URL not exposed in client bundle)
- Simpler deployment and configuration management

**Migration:**
Old: NEXT_PUBLIC_API_URL (build-time, embedded in bundle)
New: PASTEL_API_URL (runtime, read by server proxy)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

docker-compose.yml

@@ -5,17 +5,14 @@ services:
     build:
       context: .
       dockerfile: Dockerfile
-      args:
-        NEXT_PUBLIC_API_URL: ${NEXT_PUBLIC_API_URL:-http://localhost:3001}
-        NEXT_PUBLIC_APP_URL: ${NEXT_PUBLIC_APP_URL:-http://localhost:3000}
     image: ghcr.io/valknarness/pastel-ui:latest
     container_name: pastel-ui
     ports:
       - "3000:3000"
     environment:
       - NODE_ENV=production
-      - NEXT_PUBLIC_API_URL=${NEXT_PUBLIC_API_URL:-http://localhost:3001}
-      - NEXT_PUBLIC_APP_URL=${NEXT_PUBLIC_APP_URL:-http://localhost:3000}
+      # Runtime configuration - can be changed without rebuilding
+      - PASTEL_API_URL=${PASTEL_API_URL:-http://pastel-api:3001}
     restart: unless-stopped
     healthcheck:
       test: ["CMD", "curl", "-f", "http://localhost:3000/"]