Introduce a custom (non-Material) SureButton mirroring the web DS::Button: variants (primary, secondary, destructive, outline, ghost), sizes (sm/md/lg ≈ 28/36/48), optional leading widget, fullWidth, loading, and disabled. Colors resolve from the SureColors palette and the shape from radius tokens (radiusMd/radiusLg); flat custom press feedback (no Material ripple). Dimensions mirror the web DS::Buttonish::SIZES (the web composes these from Tailwind's standard scale rather than brand tokens, so they're not in sure.tokens.json).
Migrate the three main-view login buttons onto it — Sign In (primary, lg, loading), Sign in with Google (outline + SVG leading), and API-Key Login (ghost). The two buttons inside the API-key modal dialog are left for a follow-up.
Part of #2235.
