[#730] Add Remote-Node support

[yohamta]

Resolves #730

This PR implements support for managing multiple Dagu instances from a single UI, allowing users to monitor and control DAGs across different environments through a centralized interface.

Changes

• Add request proxying to support remote Dagu instances
• Implement remote node configuration via admin.yaml
• Add UI dropdown for instance selection
• Add documentation for setup

Configuration Example

# admin.yaml
remoteNodes:
  - name: "dev"                                # name of the remote node
    apiBaseUrl: "http://localhost:8080/api/v1" # Base API URL of the remote node it must end with /api/v1
  
    # Authentication settings for the remote node
    # Basic authentication
    isBasicAuth: true              # Enable basic auth (optional)
    basicAuthUsername: "admin"     # Basic auth username (optional)
    basicAuthPassword: "secret"    # Basic auth password (optional)
  
    # api token authentication
    isAuthToken: true              # Enable API token (optional)
    authToken: "your-secret-token" # API token value (optional)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 61.64%. Comparing base (a3bda7b) to head (2964137).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #731   +/-   ##
=======================================
  Coverage   61.64%   61.64%           
=======================================
  Files          53       53           
  Lines        5262     5262           
=======================================
  Hits         3244     3244           
  Misses       1764     1764           
  Partials      254      254           

Files with missing lines	Coverage Δ
internal/config/config.go	0.00% <ø> (ø)

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