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.

docs/self-hosted-drawio.md

@@ -0,0 +1,57 @@
+# Self-Hosted Draw.io Configuration
+
+In some corporate environments, `embed.diagrams.net` is blocked by network policies. This guide explains how to use a self-hosted draw.io instance with Next AI Draw.io.
+
+## Configuration
+
+Set the `NEXT_PUBLIC_DRAWIO_BASE_URL` environment variable to point to your self-hosted draw.io instance. This is a **build-time** configuration, meaning you need to rebuild the Docker image to change the URL.
+
+## Docker Build
+
+```bash
+# Build with custom draw.io URL
+docker build --build-arg NEXT_PUBLIC_DRAWIO_BASE_URL=http://your-drawio-server:8080 -t next-ai-draw-io .
+
+# Run the custom build
+docker run -d -p 3000:3000 --env-file .env next-ai-draw-io
+```
+
+## 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
+    ports:
+      - "3000:3000"
+    env_file:
+      - .env
+```
+
+## Running Draw.io Locally
+
+You can run the official draw.io Docker image:
+
+```bash
+docker run -d -p 8080:8080 jgraph/drawio:latest
+```
+
+Then build Next AI Draw.io with:
+
+```bash
+docker build --build-arg NEXT_PUBLIC_DRAWIO_BASE_URL=http://localhost:8080 -t next-ai-draw-io .
+```
+
+## Notes
+
+- The default draw.io URL is `https://embed.diagrams.net`
+- Changes to `NEXT_PUBLIC_DRAWIO_BASE_URL` require rebuilding the Docker image
+- For local development, you can set this in `.env.local`