Improve strings and layout around login and security pages

[mdcfe]

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

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