Steer modified-Enter messages without queue flash

[hamzamerzic]

Summary

• replace the two-request queue-then-force-steer shortcut from Steer composed messages with modified Enter #118 with one direct-steer request
• reserve the composed message durably before provider delivery without rendering that reserve in the queue tray
• reveal exactly one ordinary queued fallback if the live turn cannot accept the steer
• preserve Codex and Claude transcript-cut behavior as well as the separate chat-wide auto-steer preference

Prior work

#118 introduced the durable queue-then-steer shortcut and landed through #147. This is a distinct correction: the earlier design protected the message but necessarily exposed the queued intermediate state; this change uses the backend's newer reserve-before-delivery path to keep the same durability without the tray flash.

Validation

• backend steering suite: 35 passed
• frontend library suite: 1,988 passed
• frontend hook suite: 63 passed
• production frontend/PWA build passed
• git diff --check passed