docs: add gettext internationalization design
- Add comprehensive i18n design document with domain-based organization
(default, errors, auth, equipment, admin, emails domains)
- Document helper functions, implementation patterns, and migration workflow
- Fix multiple test suite issues:
* user_settings_live_test.exs: Update all tests to match current UI structure
- Fix user.name references (use first_name/last_name)
- Fix TOTP device creation (use create_totp_device/2)
- Update tab URLs and form IDs
- Update tab names (Account, API, Notifications)
- Remove obsolete tabs (Organizations, Activity)
- Fix password tests (pattern match redirect without flash token)
- Update sudo mode behavior
* account_data_controller_test.exs: Fix user profile assertions
* rate_limit_test.exs: Fix unused variable warning
* settings_live_test.exs: Fix organization form validation test
Graham McIntire
6c8e670dae