Add configurable public-facing URL setting to fix issue where invite emails
contained internal localhost addresses inaccessible to external users.
Features:
- New "Application URL" setting in System Settings (key: app.public_url)
- Real-time URL validation with visual feedback and HTTP warnings
- Test button to verify URL accessibility
- Invite preview showing actual link before sending
- Warning alerts when URL not configured
- Fallback to request-derived URL for backward compatibility
- Complete i18n support (EN, DE, ES, FR, ZH)
Backend:
- Created utils.GetPublicURL() for centralized URL management
- Added POST /settings/validate-url endpoint
- Added POST /users/preview-invite-url endpoint
- Updated InviteUser() to use configured public URL
Frontend:
- New Application URL card in SystemSettings with validation
- URL preview in InviteModal with warning banners
- Test URL button and configuration warnings
- Updated API clients with validation and preview functions
Security:
- Admin-only access for all endpoints
- Input validation prevents path injection
- SSRF-safe (URL only used in email generation)
- OWASP Top 10 compliant
Coverage: Backend 87.6%, Frontend 86.5% (both exceed 85% threshold)
Refs: #application-url-feature
