fix(chat): align mobile controls and suppress steering completion warning

[bobleer]

Changes

Treat user_steering as a normal completion in the desktop transcript, so handing off to a queued user message no longer displays the generic abnormal-ending warning. Existing failure notices remain unchanged.

Align mobile assistant prose with thinking/tool rows. Reduce send, stop, and attachment controls from 48px to 44px; use equal composer insets and align queue info/remove controls with the stop button in both expanded and collapsed layouts.

Validation

• Desktop completion notice tests: 9 passed, including steering with/without a final response.
• Real Chrome mobile queue browser tests: 3 passed, including action behavior, queue placement, button centers, equal collapsed-composer insets, and prose alignment.
• Checked 320/390/768/1200px layouts, keyboard-height viewport, dark/light themes; inspected screenshots.
• pnpm run check:web, mobile type-check/build, and pnpm run theme:color-audit:all passed.

Remote control presentation was exercised with real-browser fixtures and simulated host/relay IO. No live-device, SSH workspace, Peer Device, or Detached Dispatch end-to-end run; no transport, runtime, or persisted-shape changes.