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>

.env.example

@@ -1,8 +1,12 @@
 # Pastel API Configuration
-# URL of the Pastel API instance
-# For Docker Compose: use http://pastel-api:3001
-# For local development: use http://localhost:3001
-NEXT_PUBLIC_API_URL=http://localhost:3001
+# Server-side environment variable (runtime configuration)
+# No rebuild needed to change this!
 
-# Application URL (used for sharing and OG images)
-NEXT_PUBLIC_APP_URL=http://localhost:3000
+# For Docker Compose: use container name
+PASTEL_API_URL=http://pastel-api:3001
+
+# For local development: use localhost
+# PASTEL_API_URL=http://localhost:3001
+
+# For external API: use full URL
+# PASTEL_API_URL=https://api.example.com