feat(i18n): add internationalization framework with 10 languages

- Create i18n utility module (app/utils/i18n.py) with translation loading,
  browser language detection, AI fallback, and l10n helpers
- Add JSON translation files for EN, DE, FR, ES, IT, PT, NL, PL, ZH, RU
- Add preferred_language column to UserProfile model with migration
- Register _() translation function as Jinja2 global
- Update base.html with translated navigation, footer, cookie notice
- Add language selector dropdown in nav bar (desktop + mobile)
- Create API endpoints for language preference (POST/GET /api/i18n/)
- Support language detection: user profile > cookie > Accept-Language > default

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/models.py

@@ -256,6 +256,10 @@ class UserProfile(Base):
     preferred_destination = Column(String(50), nullable=True)
     stripe_customer_id = Column(String(64), nullable=True)
 
+    # UI language preference for i18n (ISO 639-1 code, e.g. "en", "de", "fr")
+    # NULL means "auto-detect from browser Accept-Language header"
+    preferred_language = Column(String(10), nullable=True)
+
     created_at = Column(DateTime(timezone=True), server_default=func.now())
     updated_at = Column(DateTime(timezone=True), server_default=func.now(), onupdate=func.now())