[marcel-breuer]

Summary

• Keep the per-monitoring interval selector visible for HTTP and Keyword monitoring.
• Add localized validation/help text and regression coverage for all six stale thresholds.

Closes #1030

Validation

• Targeted Pest: 44 passed, 669 assertions.
• Bun frontend check and production build passed.
• Pint and git diff --check passed.
• Full PHP analysis remains blocked by missing MCP-related classes in the existing vendor setup.