feat(app): add queued follow-up queue/steer controls for web UI

[thesamchris]

What does this PR do?

Fixes #5408

UX is inspired by Codex Desktop's queue/steer follow-up flow, adapted to OpenCode's existing app styles.

Adds queued follow-up controls and queue/steer behavior in packages/app (web + desktop):

• add a follow-up behavior setting (Queue / Steer) in General settings
• when a run is active and behavior is Queue, submitted follow-ups are staged locally instead of sent immediately
• render staged follow-ups above the composer (supports multiple queued items)
• per queued item actions: Steer, Delete, and More (Edit message, Turn off/on queueing)
• Shift+Cmd/Ctrl+Enter sends one message with the opposite behavior

Related: #12707 #8685 #6942 #4821

How did you verify your code works?

• bun run --cwd packages/app typecheck
• bun test --preload ./happydom.ts ./src/components/prompt-input/submit.test.ts --cwd packages/app
• pre-push hook also passed: bun turbo typecheck

Screenshots

1. 1 prompt running + 2 prompts queued

2. More menu (Edit message, Turn off queueing)

3. Follow-up behavior setting