Add locale runtime configuration

[klaasnicolaas]

Summary

Adds DDS-006 and DDS-006A locale support:

• configures explicit en and nl locales with English as the application default
• supports Dutch from the start through user preference, guest cookie, and browser Accept-Language detection
• resolves each request from user preference, guest cookie, browser preference, then default
• shares active and supported locale data with Inertia
• adds a profile settings language selector that saves immediately on selection
• stores users.locale directly in the initial users migration
• documents the frontend translation bundle shape for future translated UI strings

Validation

• vendor/bin/pint --dirty --format agent
• php artisan test --compact tests/Feature/LocaleConfigurationTest.php tests/Feature/LocalePreferenceTest.php
• php artisan test --compact
• npm run lint
• npm run types:check
• npm run build