feat(flowchat): show foreground send state and stabilize scroll extent

[GCWing]

Summary

• Show a foreground user message while a send is being prepared, hand it to the persisted Turn, and keep a recoverable error state when sending fails.
• Keep submission scroll intent one shot across queueing, cancellation, and the optimistic handoff.
• Stabilize the FlowChat leading extent when mounted DOM measurements differ slightly from virtualizer estimates.
• Use the shared semantic error icon and small outline recovery action.

Verification

• pnpm.cmd run check:web
• pnpm.cmd run i18n:audit
• Changed-file ESLint
• 10 focused Web UI test files: 219 tests passed
• git diff --check; merge-tree against origin/main clean

No browser or mock visual validation was performed. Native WebView visual behavior and remote workspace, remote control, peer-device, and detached-dispatch scenarios remain unexercised.

The PR contains 27 code, style, locale, and focused test files. Review, design, and process documents were excluded.