Remote Node Support

[yohamta]

Overview
Add capability to manage multiple Dagu instances from a single UI, allowing users to monitor and control DAGs across different environments or servers.

Feature Details

• Enable configuring multiple Dagu instances (remote nodes) through admin.yaml
• Add UI dropdown to switch between instances
• Support proxying API requests to remote instances
• Maintain all UI functionality across remote instances

Configuration Example

# admin.yaml
remoteNodes:
  - name: "prod"
    apiBaseUrl: "https://prod-dagu.example.com/api/v1"
  - name: "staging"
    apiBaseUrl: "https://staging-dagu.example.com/api/v1"