feat(desktop): persist composer draft image attachments (and flush draft text) across app restart

**Category:** feature request
**Surface:** Desktop (observed on 0.5.7, macOS; behavior matches current `main`)

## Problem

A composer draft with attached images does not survive an app quit or machine reboot:

1. **Queued image attachments are memory-only.** `desktop/src/features/messages/lib/backgroundMediaUploadStore.ts` keeps queued files in a module-level `Map` (`queuedAttachmentsByDraftKey`), and `useDraftPersistSnapshot.ts` is explicit about it: *"Local files cannot be persisted, so clear them at a draft-key boundary."* Since upload only happens at send time, an attached-but-unsent image is always lost on restart.
2. **Draft text is only flushed on a draft-key change.** The draft store does persist to `localStorage` (`useDrafts.ts`), but the only write path from the composer is the effect cleanup in `useDraftPersistSnapshot.ts`, which fires when the user navigates to another channel/thread or the composer unmounts. There is no flush on window close, so quitting the app (or a reboot) while still sitting in the compose box loses the text too — even though the persistence layer exists.

Repro: type a message in any channel, attach an image, quit Buzz without switching channels, relaunch → composer is empty. Switch channels first before quitting → text and already-uploaded imeta survive, the queued local image does not.

## Request

- Persist queued attachments to disk (e.g. the Tauri app-data dir, keyed by draft key, mirroring the existing `localStorage` draft schema with a file-path reference), restore them on launch, and garbage-collect on send/clear.
- Flush the current draft on window close (Tauri `onCloseRequested` / `beforeunload`) so in-place text drafts survive quit as well.

The first item is the user-facing gap; the second looks like a small win since `persistDraftEntry` already exists.

Related but distinct: #5259 covers composer text loss on iOS; this issue is about Desktop, primarily the attachment gap.

## User impact

Anyone who composes a longer post with screenshots/photos and gets interrupted by an app update, crash, or reboot loses the attachments (and usually the text) silently. For teams where the desktop app auto-updates, this happens without the user ever choosing to quit.