Feature/index trend metrics

[DevSars24]

Summary

This PR adds three small improvements to the dashboard:

• Request Rate Sparkline
• Error Rate Trend Indicator
• Keyboard Shortcuts

These are additive changes with no breaking changes and no new external dependencies.

Changes

Request Rate Sparkline

Added a small sparkline next to the total requests metric to visualize request activity over the configured lookback period.

Error Rate Trend Indicator

Added a trend indicator to the error rate metric that compares the current lookback window with the previous one and displays whether the error rate is improving or worsening when enough data is available.

Keyboard Shortcuts

Added support for the following shortcuts:

• / to focus the search box.
• Esc to close the details panel or blur the search input.
• c to copy the current request as cURL.

The shortcuts do not interfere with typing in input fields or standard browser shortcuts.

Files Updated

• Added sparkline rendering and error rate trend calculation in HtmlRenderer.cs.
• Added a configurable lookback option in DebugProbeOptions.cs.
• Added tests for the error rate trend logic in HtmlRendererTests.cs.
• Added keyboard shortcut support in debugprobe-ui.js.

Testing

• Verified the sparkline renders correctly.
• Verified the error rate trend updates as expected.
• Verified all keyboard shortcuts work correctly.
• All existing tests continue to pass.