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

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

Look at the webview composer component, likely in a file like src/webview/composer.ts or similar. Examine the existing styling and structure. The redesign involves creating a rounded bordered card with a borderless textarea and a bottom control row with optional settings slot and Send/Stop buttons. Reference Goose Desktop composer for visual guidance. Test changes in the webview to ensure the new design matches the mockup.

Escrita pelo modelo de indexação a partir do texto da issue.