Sync unsent composer text across devices

[josephv123]

Problem

What I've typed in a composer but haven't sent yet stays on the device where I typed it. If I start a prompt on desktop and pick up my phone (or open app.t3.codes on another machine), the box is empty on that other client.

Today composer drafts are stored per client:

• Web/desktop: apps/web/src/composerDraftStore.ts persists to localStorage.
• Mobile: apps/mobile/src/state/use-composer-drafts.ts writes a drafts file on the device.

Neither one goes through the server, so a draft never reaches another client connected to the same environment. That's at odds with how the rest of a thread works, since messages, queue, and status already sync everywhere.

Proposed behavior

The unsent text in a thread's composer syncs across every client connected to that environment. If I type on desktop, then open the same thread on mobile, I see the same text and can keep editing it or send it from there.

• Scope it to the composer of an existing thread first. That's the common "started typing at my desk, finish on my phone" case.
• Last write wins, with a short debounce. No live co-editing or cursor sync, because two people typing in the same box at once isn't a real use case.
• Sending, or clearing the box, clears the draft everywhere.
• Small payloads only (text plus references to already-uploaded attachments), so it doesn't add meaningful websocket traffic.

Smallest useful scope

Text only, for existing threads, stored server-side per thread and pushed to subscribed clients. Leave attachments, model/trait selections, and new-thread drafts for later.

Open questions

• Should this be on by default, or a setting? Some people may not want half-written prompts showing up on another device.
• Where does it live? It could be a field on the thread projection (event-sourced) or a lighter key/value store outside orchestration, since keystroke-level drafts probably shouldn't become events.

Related

#6895 proposes modeling unsent drafts as real threads and mentions server-side draft text (thread.draft.set) as a follow-up step that would give cross-device sync. This discussion is narrower: it only asks for the composer contents to sync, without changing how threads or the sidebar work.