feat(i18n): add Mexican Spanish localization

[e-macgregor]

Summary

• add a complete Mexican Spanish (es-MX) frontend catalog
• add Mexican Spanish labels for generated invoice documents, including RFC and IVA terminology
• register es-mx in the frontend and backend and expose it in the language selector

Testing

• catalog parity check: 295 source keys, 0 missing, 0 extra, 0 placeholder mismatches
• frontend locale resolution smoke test
• deno fmt --check, deno lint, deno check, and deno test for the backend localization files
• bun run build
• bun run check (0 errors; 4 pre-existing warnings)
• docker build .

Scope

This adds an opt-in locale only. It does not change Invio's global defaults and does not add or claim CFDI/SAT functionality.