fix: remove duplicate CH region code in RegionCode type

[lamborghini21]

Summary

Fixes #715 — duplicate CH region code in src/locales/types.ts was
causing silent overwrite during locale resolution for Switzerland.

Changes

• Removed duplicate CH entry under German regions in RegionCode type
• Replaced it with LI (Liechtenstein) which is the correct German-speaking region
• Added unit test in src/locales/config.test.ts to assert all region codes are unique

Testing

• pnpm test src/locales/config.test.ts → ✅ 1 passed
• pnpm run check-locales → ✅ exits with code 0

Acceptance Criteria

• RegionCode contains no duplicate values
• check-locales script exits with code 0
• Swiss region resolves to the expected locale

Closes #715

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