feat: add Key language setting for non-US scancode labels

[mondalaci]

Summary

• Adds an Agent-only Key language setting (US / UK / German / Nordic) on the Settings page above Theme, persisted in application settings (not UHK user config).
• Unifies scancode→symbol labels in uhk-common and drives keymap SVG legends plus keymap/macro scancode dropdowns from the selected language.
• Updates the scancode tooltip to reference the new setting. Closes Non-US layout support #2147.

Test plan

• Open Settings and switch Key language between US, UK, German, and Nordic; confirm the choice persists after restart
• On the keymap page, confirm key legends update for language-specific keys (e.g. German Ö/Ä/Ü and Y/Z; Nordic Å/Ö/Ø/Ä/Æ)
• Open a key remap popover and a macro key action; confirm scancode dropdown labels match the selected language and search finds the new labels
• Confirm ANSI/ISO hardware layout behavior is unchanged
• Confirm Theme and other Agent settings still save normally

Made with Cursor

[yetisbey]

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