fix: improve SSO-only login page layout

[TerrifiedBug]

Summary

• Remove empty CardContent/CardFooter gap when local authentication is disabled, eliminating the large dead space in the SSO-only login card
• Center text and add a shield icon badge as a visual anchor
• Consolidate the SSO button into CardContent (removing separate CardFooter) for a compact layout

Test plan

• Set LOCAL_LOGIN_DISABLED=true and OIDC_ENABLED=true — verify the login card is compact with centered text and shield icon
• Verify the SSO button still works correctly
• Set LOCAL_LOGIN_DISABLED=false — verify the standard login form is unchanged
• Trigger an SSO error and confirm the error message renders centered within the card

[greptile-apps]

Greptile Summary

This PR is a focused UI polish for the SSO-only login page (when local auth is disabled and OIDC is enabled). It eliminates the dead space that previously existed between CardContent and CardFooter by consolidating the SSO button into CardContent and removing the separate CardFooter. A centered shield icon badge and centered text are added as visual anchors. The standard login form path is completely untouched.

• Moves <Button> from <CardFooter> into <CardContent> to eliminate the layout gap in the SSO-only branch
• Adds a Shield icon badge in CardHeader as a visual anchor (centered)
• Centers CardHeader text and the error message in the SSO-only branch
• Adds pt-0 to CardContent and pb-4 to CardHeader for compact spacing
• No functional, logic, or security changes — the standard login form, TOTP flow, and SSO callback handling are all unaffected

Confidence Score: 5/5

• This PR is safe to merge — changes are purely cosmetic and scoped to a single conditional UI branch.
• The diff touches only Tailwind class strings and JSX structure within the SSO-only rendering branch. No logic, state, authentication flow, or API calls were modified. The standard login form, TOTP handling, and SSO callback are all untouched. No new imports were added and no existing ones became unused.
• No files require special attention.

_{Last reviewed commit: c43ab60}