fix(composer): address image-editor follow-up nits on #1491

[tellaho]

Category: improvement
User Impact: Image annotation opens faster for large uploads and behaves more consistently if a save is already in progress.
Problem: Follow-up review on #1491 called out a few sharp edges in composer image editing: fetched image bytes were serialized as JSON arrays, Escape could look like cancel while a save still completed, and the revert affordance’s draft behavior needed an explicit scope decision.
Solution: Return fetched image bytes over Tauri IPC as a raw buffer, gate Escape while the editor is saving, and document the in-memory-only revert behavior as the intended follow-up boundary.

File changes

desktop/src-tauri/src/commands/media_download.rs
Returns image-editor media bytes as tauri::ipc::Response so large images cross IPC as raw bytes instead of JSON number arrays while preserving existing validation and size caps.

desktop/src/features/messages/lib/useMediaUpload.ts
Documents the revert-originals map as intentionally in-memory-only, recording draft round-trip persistence as outside this follow-up’s scope.

desktop/src/features/messages/ui/ComposerAttachments.tsx
Tracks the image editor’s save state in the lightbox and prevents Escape from leaving edit mode while a save upload is in flight.

desktop/src/features/messages/ui/ComposerImageEditor.tsx
Reports save-in-flight state to the parent and clears that signal on unmount so the lightbox Escape guard stays accurate.

desktop/src/shared/api/tauriMedia.ts
Reads fetch_media_bytes responses as an ArrayBuffer and wraps them in a Uint8Array for the editor.

desktop/src/testing/e2eBridge.ts
Updates the E2E mock for fetch_media_bytes to return the same raw ArrayBuffer shape as the Tauri command.

Reproduction Steps

1. Open the desktop composer and attach an image.
2. Open the attachment lightbox, enter draw mode, add a stroke, and save.
3. Confirm the edited attachment replaces the original and can still be reverted while the composer remains active.
4. Start another edit and press Escape before saving; the editor should leave draw mode normally.
5. While a save is in progress, Escape should no longer make the UI look canceled while the attachment swap still lands.

Validation

• GitHub CI: previously green after rerunning one unrelated smoke failure; latest run for 770c71ef is in progress with no failures observed yet.
• Marge final gate: approved in-thread for focused scope/correctness; follow-up spoiler active-state re-check passed on 770c71ef with independent screenshot verification.
• Local checks from final gate: biome check ., file-sizes, px-text, tsc, pnpm test 1885/1885, desktop-tauri fmt-check, and cargo check passed; local clippy findings were pre-existing outside this PR.

Screenshots / Mocks

Editor lightbox idle	Editor lightbox active annotation

Spoiler inactive	Spoiler active

Coordination

buzz://message?channel=99a4d0c9-e02b-49e3-aa81-e637c4fd0292&thread=790c65884f4b236b88f23b9c7a82dd904d4113a8822bc1c8df3c4f7896896ff8