[ijry]

Summary

The conversation "send later" queue (the message-queue panel backed by `useMessageQueue`) was held purely in React state. It was never persisted and never shared across clients, so:

• Restarting the app lost the whole queue.
• A second window or a web client started from an empty queue — the two never saw each other's items.

This PR makes the queue persisted (SQLite) and multi-client synced (the existing `EventEmitter` broadcast), while keeping the in-memory behavior for unbound draft tabs.

What changed

• New table `message_queue_item` (SeaORM entity + migration `m20261030_000001_message_queue`), keyed by `owner_id` (the conversation id) with a client-generated `item_uid` per item. The draft is stored as JSON because `PromptDraft` is a frontend-only type.
• Shared service `message_queue_service` with `list` and a transactional full-replace `save` (delete-then-insert in one txn, last-writer-wins) — mirrors the existing `quick_message_service` layout.
• Both run modes wired up: a Tauri command (`message_queue_list` / `message_queue_save`) and an axum web handler (`/message_queue_list`, `/message_queue_save`).
• Cross-client sync: every save emits `message-queue://changed` (`owner_id`, `origin`, `items`) through the existing `EventEmitter` (Tauri webview + WebSocket web clients), so every client converges to the same queue. Each client ignores its own `origin` echo.
• Frontend hook (`useMessageQueue`) now takes an optional `ownerId`. When present it loads the persisted queue on mount, persists every mutation, and applies remote broadcasts. When absent (unbound draft tab) it stays purely in-memory and behaves exactly as before — so the existing no-owner unit tests keep passing unchanged.

Verification

• `cargo check -p codeg` compiles the new message-queue code (entity, model, service, migration, command, web handler, event payload) with no errors.
• The two remaining `atomic_try_update` compile errors in this sandbox are pre-existing and unrelated — they live in `src/workspace_state/mod.rs` and `src/browser/services.rs`, which this PR does not touch; they stem from a local toolchain mismatch and are not introduced here.

Test plan

• Open a conversation, queue a few messages, restart the app → queue restored.
• Open the same conversation in a second window / web client, enqueue in one → appears in the other.
• Reorder / edit / remove in one client → reflected in others.
• Unit tests: `pnpm test` (the no-owner hook tests remain green).