style(webview): composer card redesign with bottom control row

[cloud-on-prem]

Extracted from #39. Restyle the composer as a rounded bordered card (input background, inset highlight) containing a borderless transparent textarea and a bottom control row: an optional settings slot on the left, Send/Stop on the right. Mirrors the Goose Desktop composer.

Keep the settings slot optional so this does not depend on the mode/model controls issue (#67).

Size: S-M | Depends on: nothing | Tier: 2