Polish Huddle participant interactions

[klopez4212]

Summary

• arrange Huddle participants in a responsive, equal-weight cluster with spring enter/exit motion and a +N overflow
• spotlight tapped participants over a blurred call surface, with a roster for hidden participants and no self-avatar action
• add selection haptics across full-screen and drawer controls, including both end-call buttons

Validation

• just mobile-check
• focused participant, drawer-control, and full-screen end-call widget tests
• Huddle-focused widget suite (15 tests)
• full mobile Flutter suite (1,538 tests)

Dependency

Built on #6056 and contains only the follow-up interaction work. Merge after #6056 lands.