[I18N-01] Complete ES, EN and PT interface translations

[Krisor1986]

Problem

Only the dashboard and a small part of the navigation reacted to the language selector. Authentication, plans, clients, leads, jobs, calendar, conversations, knowledge, agents, team management and validation feedback continued mixing Spanish and English.

Result

• centralizes complete English, Spanish and Portuguese interface catalogs
• translates public and authenticated flows, including forms, filters, statuses, validation and error feedback
• exposes the language selector on authentication and invitation screens
• applies language changes immediately and persists the selection across reloads
• updates dates, months and weekdays using the active locale
• warns during development and tests when a catalog entry is missing

Validation

• npm test — 9 passed
• npm run lint
• npm run build
• backend suite — 201 passed, 2 skipped
• manual ES → EN → PT switch without reload and Portuguese persistence after reload

Closes #18