feat: add shared English and Italian localization

[cmdr-chara]

Summary

Adds initial English/Italian localization support to Quickshell Rise for both V1 and V2.

What changed

• Added a shared Translation.qml localization helper
• Detects locale using LC_ALL, LC_MESSAGES, then LANG
• Italian locales use it_IT; unsupported locales fall back to English
• Localized:
  • Clock tooltip dates
  • Calendar month and weekday labels
  • Weather panel labels and forecast days
• Removed hardcoded English month/day arrays where replaced by locale-aware formatting
• Added qs-i18n-regression.sh
• Updated usage and development documentation

Scope

This PR intentionally establishes the localization infrastructure with a limited first set of UI surfaces rather than translating the entire shell at once.

The shared helper can be extended later for other panels and additional languages without duplicating locale logic between V1 and V2.

Testing

• Added static regression checks for both V1 and V2 localization paths
• Verified the branch is based directly on current upstream main
• No unrelated workflow/bootstrap files are included