Add an Enter-to-queue, Cmd/Ctrl+Enter-to-steer follow-up mode

[felipeorlando]

Problem

Merged upstream PR hardbeat920#56 added a global Follow-up behavior setting with two mutually exclusive choices: Queue or Steer. While a turn is running, plain Enter always uses that selected behavior, and the composer does not inspect Cmd/Ctrl modifiers when submitting.

I want a third option that keeps both actions one keystroke away:

• Enter queues the follow-up.
• Cmd+Enter on macOS / Ctrl+Enter on Windows and Linux steers the active turn immediately.
• When the session is idle, both shortcuts send a normal new turn.
• Shift+Enter continues to insert a newline.

Proposed behavior

Add a Queue, shortcut to steer choice under Settings → General → Follow-up behavior. In that mode, pass the keyboard intent through the existing submit path instead of adding a second queue implementation.

The Send button can keep the default Queue action while busy; the existing Steer action on queued rows remains available.

Existing work checked

• Upstream PR feat: add configurable queued follow-ups hardbeat920/monocode#56 implemented queue state, automatic dispatch, explicit Steer actions, and the current Queue/Steer setting.
• No upstream issue or PR adds a modifier-based per-message override.

Acceptance criteria

• With the new mode selected and a turn running, Enter queues and Cmd/Ctrl+Enter steers.
• With no active turn, both shortcuts send normally.
• Shift+Enter still inserts a newline and composition/command-picker behavior remains intact.
• Existing Queue-only and Steer-only modes keep their current behavior.
• The setting persists across restarts.