Add possibility to change timestamp format

[tom2drum]

Description and Related Issue(s)

Resolves #3048

This PR adds the ability to change timestamp format throughout the application, similar to beaconcha.in. Users can now toggle between local time and UTC time formats globally via site settings, and switch between local/UTC/Unix formats on detailed views.

Proposed Changes

• Global time format setting: Added a new "Local time format" toggle in site settings (default: ON) that allows users to switch between local and UTC time format globally across the application
• Detailed view time format control: Added a time icon selector on detail pages that allows users to switch between Local/UTC/Unix formats on a per-page basis, independent of the global setting
• Component updates: Updated all time display components across the application (tables, list items, detail pages, search results, etc.) to respect the global time format setting
• Uptime graphs: Adapted uptime charts to work with UTC time from settings
• Tooltip enhancements: Time tooltips now show full time format including timezone information (e.g., "Jun 23 2022 02:51:55 PM (+04:00 UTC)")

The implementation uses cookies to persist user preferences and integrates seamlessly with the existing settings context.

Breaking or Incompatible Changes

None.

Additional Information

This implementation follows the design specifications from the Figma mockups referenced in issue #3048. The mobile version shows full timeline format without additional controls, as specified in the requirements.

Checklist for PR author

• I have tested these changes locally.
• I added tests to cover any new functionality, following this guide
• Whenever I fix a bug, I include a regression test to ensure that the bug does not reappear silently.
• If I have added, changed, renamed, or removed an environment variable
  • I updated the list of environment variables in the documentation
  • I made the necessary changes to the validator script according to the guide
  • I added "ENVs" label to this pull request