Refine web UI Nexu component usage

[mrcfps]

Summary

• Replace more bespoke web UI controls with Nexu primitives across the shell, composer, chat thread, and settings panels.
• Align global base styles with the Nexu demo by removing broad form-control and hover overrides and adopting demo-style scrollbars.
• Refine model/provider controls, including compact catalog/test actions, destructive delete styling, and textarea placeholder alignment.

Verification

• pnpm --filter @monet/web-ui typecheck