feat: add support for self-hosted draw.io via DRAWIO_BASE_URL environment variable (#163)

## Summary
Add support for self-hosted draw.io instances via build-time configuration.

## Problem
In some corporate environments, `embed.diagrams.net` is blocked by network policies. 
Users cannot use the application without access to the default draw.io embed URL.

## Solution
- Add `NEXT_PUBLIC_DRAWIO_BASE_URL` environment variable support
- Pass the `baseUrl` prop to the `DrawIoEmbed` component
- Configure Dockerfile to accept build-time argument for the draw.io URL

## Usage
```yaml
# docker-compose.yaml
services:
  drawio:
    image: jgraph/drawio:latest
    ports:
      - "8080:8080"
  
  next-ai-draw-io:
    build:
      context: .
      args:
        - NEXT_PUBLIC_DRAWIO_BASE_URL=http://drawio:8080
```

Or build directly:
```bash
docker build --build-arg NEXT_PUBLIC_DRAWIO_BASE_URL=http://localhost:8080 -t next-ai-draw-io .
```

**Note:** This is a build-time configuration. To change the draw.io URL, you need to rebuild the Docker image.

env.example

@@ -60,3 +60,7 @@ AI_MODEL=global.anthropic.claude-sonnet-4-5-20250929-v1:0
 
 # Access Control (Optional)
 # ACCESS_CODE_LIST=your-secret-code,another-code
+
+# Draw.io Configuration (Optional)
+# NEXT_PUBLIC_DRAWIO_BASE_URL=https://embed.diagrams.net  # Default: https://embed.diagrams.net
+# Use this to point to a self-hosted draw.io instance