Wire `SyncEngine::upload_bytes` to a Settings → Sync button:
- `BrowserCore::build_sync_snapshot_bytes` serialises the user's
bookmarks on the UI thread (cheap, synchronous).
- `ElyShell::trigger_cloud_sync_upload` resolves the active
profile data dir, spawns a dedicated `ely-sync-upload` thread,
and lets the engine run the blocking HTTP round-trip there so
the GPUI render loop never stalls on the network — the same
invariant the Servo IPC worker enforces.
- Outcomes go through `tracing` on the `ely::sync` target. Users
drop a Better Auth bearer token into
`<profile_data>/sync/bearer.token` to opt in; without one, the
engine reports `SignedOut` and the click is a no-op.
The Better Auth handshake + device-approval UX still need their
own UI passes; this lands the data-plane plumbing so those pieces
slot in without re-architecting the snapshot path.
ZacharyZhang-NY
6bacb3faa8