Clean up i18n namespaces and localize LN invoice copy feedback

[pepakriz]

Motivation

• Align registered i18n namespaces with the actual locale modules to avoid stale/unused namespace entries.
• Replace a hardcoded English toast message with a translation key so the LN-copy feedback is localizable.
• Remove an unused locale file that was no longer referenced by the resources wiring.

Description

• Removed the obsolete clients namespace and added the missing transactions namespace in lib/i18n/config.ts to match loaded resources.
• Replaced the hardcoded copy-confirmation string in components/receive-payment.tsx with t("payments:detail.messages.ln-invoice-copied-to-clipboard").
• Added the new translation key payments:detail.messages.ln-invoice-copied-to-clipboard to locales/en/payments.ts and locales/cs/payments.ts.
• Deleted the unused file locales/en/clients.ts as it was not referenced by the resources setup.

Testing

• Ran a parity check script comparing locales/en and locales/cs which reported no unmatched files (onlyEn [], onlyCs []).
• Ran bun run check:lint which completed successfully (existing warnings remain but no new lint errors introduced).
• Ran type checks with bun run check:types (tsc) which failed due to pre-existing TypeScript errors in app/(client)/settings/page.tsx unrelated to these changes.

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Codex Task