fix(prompt-input): unify steer/queue composer into a codex-style morphing button

[astyfx]

Summary

Redesign the Steer/Queue composer UX to match the Codex app / beui agent-chat-input style: one morphing Send/Stop button, no floating dual-key chrome, and prettier floating queued-turn cards.

Changes

Fixes

• prompt-input: replace the separate abort (OctagonX) button and the primary Send button with a single button that morphs to a filled Stop square when a turn is running and the composer is empty, and back to Send/Queue once text is present
• prompt-input: remove the floating "Adjust current work" secondary button and its verbose dual-key tooltips/placeholders; keep the steerQueueEnterAction setting and the Enter/Tab shortcut behavior fully intact
• prompt-input: add Esc-to-stop while composing, so the turn can always be interrupted even when the composer has text and the button face shows Send
• prompt-input: restyle queued-turn cards into lighter floating cards (rounded, soft shadow, backdrop blur, hover-revealed edit/delete controls, highlighted "Next to send" card) using semantic theme tokens for light/dark

Refactors / Chores

• Remove now-unused steerQueueSecondary* locals and the CornerUpLeft/OctagonX imports; add Square

Test Evidence

• bun run typecheck passes clean
• Manually verified via a throwaway Vite harness mounting the real PromptInput component (removed after verification), driving real DOM/keyboard events:
  • Idle: Send disabled when empty, enabled with text
  • Running + empty composer: Stop button appears (destructive styling, filled square icon), clicking calls onAbort
  • Running + text: button flips back to Send/Queue; Esc still calls onAbort even with a draft present (draft preserved)
  • Enter/Tab still submit with the correct queue/steer intent per the configured steerQueueEnterAction
  • Queued cards: edit, delete, clear-all, and "Next to send" marker all work; dark mode toggle produced no console errors (semantic tokens only)

Notes

Pure renderer UI/interaction change — steering/queue backend (sendUserMessage intents, steerTurn env gating, auto-flush) is untouched.