Add persistent network switcher with localStorage

[soma-enyi]

Closes #104

Persistent Network Switcher for Status Monitor

Overview

Added a network switcher to the status monitor dashboard that allows developers
to quickly toggle between different Stellar networks during testing and
integration work.

Features

• Toggle Buttons: Three-button switcher in the top navbar (Testnet,
  Mainnet, Local)
• Persistent Selection: Uses localStorage to remember network choice across
  page reloads
• Dynamic Configuration: Automatically updates base URL and re-checks
  endpoints when network changes
• Visual Feedback: Active network button highlighted with blue styling

Network Configurations

• Testnet: https://horizon-testnet.stellar.org
• Mainnet: https://horizon.stellar.org
• Local: http://localhost:8080 (default)

Implementation

• Pure JavaScript with no dependencies
• localStorage key: anchorkit_network
• Auto-triggers endpoint health checks on network switch
• Responsive navbar design

Benefits

• Faster Testing: Switch networks with one click instead of manually
  editing URLs
• Improved Workflow: No need to remember different endpoint URLs
• Persistent State: Network selection survives page refreshes
• Developer Experience: Streamlines integration testing across environments

Usage

1. Open status monitor in browser
2. Click desired network button (Testnet/Mainnet/Local)
3. Selection is saved automatically
4. Endpoints are checked immediately
5. Reload page - last selection is restored

Files Changed

• status-monitor.html - Added navbar, network switcher UI, and localStorage
  logic