IAV-951/ UI improvements, auth flow fixes, and marketplace as default entry
- Replace browser alerts/confirm/prompt with glassmorphism dialog popups for delete, rename, and view file actions in AgentTrainingSection
- Add inline PDF viewer (toolbar hidden), text and image preview in modal
- Fix config tabs scrolling: toggle stays fixed, content scrolls independently
- Show info message when no accounts exist instead of empty dropdown (legal integration modal + aqal wizard)
- Fix post-login blank page: guards show Loading instead of returning null
- Remove redundant AuthGuard in dashboard page
- Fix double loading after login: LOGIN action now sets loading to false
- Redirect /dashboard to /dashboard/marketplace as main entry point (server-side redirect, post-login, and guest-guard fallback)