## What changed
- switch the cloud sandbox upload client to a multipart
manifest-plus-file request instead of JSON `Record<string, string>`
payloads
- read synced files as raw bytes for both full and incremental uploads
while preserving `replaceAll`, `deletedFiles`, and queued incremental
sync behavior
- update the fake cloud sandbox server and desktop-side tests to
validate multipart uploads and binary byte preservation

## Why
The previous upload path decoded files as UTF-8 strings before sending
them to the engine, which corrupted binary assets and broke cloud
previews for files like images, fonts, and wasm.

## Impact
Desktop cloud sandbox syncs now send byte-exact file contents to the
engine, including incremental syncs.

## Compatibility
- depends on the engine multipart upload support in
dyad-sh/dyad-llm-engine#146
- the engine PR keeps legacy JSON uploads available for older desktop
clients, but this desktop change is required for binary-safe uploads

## Validation
- `npm test -- src/ipc/utils/cloud_sandbox_provider.test.ts`
- `npm run ts`
- `npm run lint`
- `npm run fmt:check`

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Co-authored-by: Will Chen <7344640+wwwillchen@users.noreply.github.com>