fix(chat): restore queued steering on enter

[morgmart]

Summary

• restore the double-enter flow: while an agent is working, a send shortcut with an empty composer steers the first visible queued message
• keep steering inert for drafts, queued edits, hidden queue heads, idle sessions, disabled sends, and in-flight attachment admission
• restore the durable chat laws and regression coverage that were missed during the OSS repository migration

Context

This completes the behavior from #3, which was merged into the temporary kalvin/initial-oss-port branch instead of main. Most supporting guards later reached main through #7; the ordinary chat.sendMessage route, laws, and focused tests did not.

Testing

• pnpm vitest run src/features/chat/ui/__tests__/ChatInput.test.tsx src/features/chat/ui/__tests__/ChatInput.attachments.test.tsx — 158 passed
• just check
• pnpm typecheck
• pnpm test — 531 files, 6,205 passed, 1 skipped
• pre-push fmt-check, check, tauri-check, and clippy