Improve strings and layout around login and security pages

[mdcfe]

A handful of minor layout and text improvements - nothing drastic in this PR

### Tasks
- [x] `/auth/settings/security`: Fix capitalisation and add icons for Google and Microsoft OAuth buttons
- [x] `/auth/settings/security`: Improve wording around 2FA methods and backup codes
- [x] `/auth/login`: Use a dynamic card header for the 2FA prompt, not just the sudo prompt
- [x] `/auth/login`: Improve 2FA flow - make strings translatable and fix capitlisation
- [x] `/auth/login`: Improve 2FA flow - don't immediately show backup code prompt alongside TOTP prompt
- [x] Navbar: Change "Login" and "Signup" buttons to "Log in" and "Sign up"