chore: session-ui prompt-input uses \\200B CSS escape that oxlint reports as an error, breaking bun run lint

Description

packages/session-ui/src/v2/components/prompt-input/index.tsx uses empty:before:content-['\200B'] (a CSS hex escape for zero-width space) inside a JSX class attribute. oxc's parser reports this as a JS octal-escape early error, making bun run lint exit 1 on dev for everyone.

The string is consumed by CSS verbatim (no JS unescaping happens in a JSX attribute string), so the escape is correct at runtime — the two are just in conflict.

Steps to reproduce

1. bun run lint at dev
2. lint exits 1: "'0'-prefixed octal literals and octal escape sequences are deprecated" at prompt-input/index.tsx

Fix incoming in PR #47510 (class moved to a JS expression with the escape written for the JS parser — emitted string verified byte-identical); filing per the Issue First policy so the drive-by fix is referenceable.