Keep composer actions aligned while stopping (CL-7293)

[pratikbuilds]

Summary

Keeps Stop and Send in one right-aligned action group while a turn is running.

Send remains available for a queued follow-up while Stop cancels the active turn.

Verification

• bun run check
• Composer regression coverage for grouped actions and queued sends

Fixes #542

Fixes CL-7293

[TheGreatAxios]

Groups Stop and Send under a single grid cell so the 3-column .chat-composer-actions grid doesn't misalign when a 4th child (Stop) appears during a running turn — clean, targeted fix with a plausible root-cause story matching the bug it closes (#542).

Must fix

None.

Consider

• packages/chat-ui/src/composer.tsx: aria-label/title for the Send button still branch only on sending (sending ? composerSending : composerSend), unchanged by this PR but now visually adjacent to Stop in the same group — worth double-checking that a screen reader user gets a clear enough label distinguishing "Send" from "Stop" when both render side by side (both are icon-only buttons).
• packages/chat-ui/src/styles.css: .chat-composer-submit-actions gap (0.25rem) is a new ad-hoc value; it's consistent with the file's existing convention of literal rem values (no spacing custom properties exist in this file), so not a blocker, but flagging in case a token system is introduced later.

Verified

• gap: 0.25rem and other literal rem spacing values match the existing convention throughout styles.css (no CSS custom-property spacing tokens are used anywhere in this file) — not a DESIGN.md violation.
• Both buttons still use the shared Button component and @corbits/icons icons from @corbits/react-ui — no duplicated primitive.
• The wrapping <div className="chat-composer-submit-actions"> correctly reduces .chat-composer-actions back to exactly 3 direct children (attach, keyboard hint, submit-actions), matching the grid's 1fr auto 1fr template regardless of whether Stop is rendered — this is the actual fix for the alignment bug.
• New tests (keeps Stop and Send together..., keeps queued sends available while Stop is visible) exercise the DOM structure and the queued-send behavior directly rather than just re-asserting old expectations; CI (build-test, e2e, isolation, lint, structural, typecheck, db-suites) is all green.

[pratikbuilds]

Reviewed all feedback on this PR.

• Added regression coverage confirming that the adjacent icon-only controls expose distinct accessible names: Stop and Send.
• Kept the 0.25rem action-group gap unchanged because it follows the existing literal-rem spacing convention in this stylesheet.

The review had no must-fix items.

Verification: bun test test/composer.test.tsx (25 passing) and bun run check.