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.

app/page.tsx

@@ -11,6 +11,9 @@ import {
 } from "@/components/ui/resizable"
 import { useDiagram } from "@/contexts/diagram-context"
 
+const drawioBaseUrl =
+    process.env.NEXT_PUBLIC_DRAWIO_BASE_URL || "https://embed.diagrams.net"
+
 export default function Home() {
     const { drawioRef, handleDiagramExport, onDrawioLoad } = useDiagram()
     const [isMobile, setIsMobile] = useState(false)
@@ -109,6 +112,7 @@ export default function Home() {
                                     ref={drawioRef}
                                     onExport={handleDiagramExport}
                                     onLoad={onDrawioLoad}
+                                    baseUrl={drawioBaseUrl}
                                     urlParameters={{
                                         ui: drawioUi,
                                         spin: true,