Add dashboard-style console UI with ratatui for real-time monitoring

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This PR implements a comprehensive dashboard-style console UI using ratatui that provides real-time visualization of monitored URLs and their status, addressing the need for better observability and usability when monitoring multiple endpoints.

🎯 Key Features

Interactive Terminal Dashboard

• Real-time status updates for all monitored services without manual refresh
• Color-coded health indicators (Green for Healthy, Red for Unhealthy)
• Response time display in milliseconds for successful requests
• Human-readable countdown to next check (e.g., "2m15s", "30s")
• Error message display with smart truncation for failed requests

Advanced Filtering & Sorting

• Filter modes: All → Healthy Only → Unhealthy Only (toggle with f)
• Sort modes: URL → Status → Response Time → Next Check (toggle with s)
• Vim-like navigation: k/↑ (up), j/↓ (down), q/Esc (quit)

Dual Operation Modes

The application now supports two operation modes with zero breaking changes:

Headless Mode (Default)

./fluxa --config config.toml

Dashboard Mode (New)

./fluxa --config config.toml --dashboard
# or shorthand:
./fluxa -c config.toml -d

🔧 Technical Implementation

Architecture Changes

• Extended MonitoredService struct with dashboard fields (response_time, error_message, next_check, last_check)
• Created dedicated dashboard module with clean separation from core monitoring logic
• Implemented tokio channels for real-time communication between monitoring tasks and UI
• Added proper terminal cleanup and graceful error handling

Dependencies Added

• ratatui = "0.29" - Professional terminal UI framework
• crossterm = "0.28" - Cross-platform terminal manipulation
• Enhanced tokio with sync feature for channels

Dashboard Layout

┌Status────────────────────────────────────────────────────────────────┐
│Fluxa Dashboard - Filter: All | Sort: Url | Services: 3               │
└──────────────────────────────────────────────────────────────────────┘
┌MonitoredServices─────────────────────────────────────────────────────┐
│URL                    Status      Response Time    Next Check  Error │
│https://example.com    Healthy     150.23ms        2m15s       -     │
│https://api.service.com Unhealthy  N/A             5s          HTTP…  │
└──────────────────────────────────────────────────────────────────────┘
┌Help──────────────────────────────────────────────────────────────────┐
│q/Esc: Quit | f: Filter | s: Sort | ↑/k: Up | ↓/j: Down              │
└──────────────────────────────────────────────────────────────────────┘

✅ Quality Assurance

• All existing tests pass (7/7) - no regression in functionality
• Release build successful - production ready
• Memory efficient - minimal overhead for dashboard features
• Backward compatible - existing configurations and usage unchanged
• Professional error handling - graceful degradation and cleanup

📚 Documentation

Updated README.md with comprehensive usage instructions including:

• Clear examples for both headless and dashboard modes
• Complete key binding reference
• Filter and sort mode explanations
• Visual dashboard layout example

This implementation significantly improves the monitoring experience by providing real-time visibility into service health, response performance, and upcoming checks while maintaining the lightweight, efficient nature of the original application.

Fixes #47.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• api.pushover.net
  • Triggering command: target/debug/fluxa --config config.test.toml --dashboard (dns block)
  • Triggering command: target/debug/fluxa --config config.test.toml (dns block)
• httpbin.org
  • Triggering command: target/debug/fluxa --config config.test.toml --dashboard (dns block)
  • Triggering command: target/debug/fluxa --config config.test.toml (dns block)
• this-url-does-not-exist-12345.com
  • Triggering command: target/debug/fluxa --config config.test.toml --dashboard (dns block)
  • Triggering command: target/debug/fluxa --config config.test.toml (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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