fix(flow-chat): keep composer stable across queue changes

[bobleer]

Summary

• keep the composer input surface geometry identical whether queued messages exist or not
• remove dynamic :has() / :empty styling that could leave a stale double shell after automatic dequeue
• add a regression test that locks the queue-independent composer contract

Validation

• pnpm run design-system:check
• pnpm run check:web
• pnpm --dir src/web-ui run test:run src/flow_chat/components/PendingQueuePanel.test.tsx (5 tests)
• pnpm --dir src/web-ui exec vitest run --maxWorkers=50% src/flow_chat/utils/flowChatScrollLayout.test.ts src/flow_chat/components/modern/flowChatTailFollow.test.ts (43 tests)

Scope

Presentation-only change. No remote-workspace, remote-control, Peer Device Mode, or Detached Dispatch behavior is changed.