feat(login): redesign for multi-forge with segmented forge selector

[afonsojramos]

Summary

Redesigns the login screen now that #2842 introduced multi-forge support, surfacing the forge as the meaningful first choice and demoting login methods to a clearer primary/alternate hierarchy.

• Segmented forge selector with a sliding pill indicator (useLayoutEffect + measured ref) that animates between tabs. Scales linearly to N forges via the adapter registry.
• Primary CTA per forge — adapter's variant: 'primary' method (or first registered) becomes "Continue with X"; alternates render as quieter invisible buttons under the primary.
• Per-forge tagline added as optional ForgeAdapter metadata so each adapter ships its own copy ("github.com & GitHub Enterprise", "Self-hosted Git service") without bleeding into shared UI code.
• Compact layout that fits inside the 500×400 menubar window — no overflow on any forge configuration.
• Entrance animations (staggered fade-up + cross-fade on forge change) defined in tailwind.config.mts keyframes; honour prefers-reduced-motion via motion-reduce: variants.

Screen.Recording.2026-05-08.at.14.57.08.mp4

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
95.8% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[setchy]

🚀