Add per-profile sync orchestration to `ely_browser_core`:
- `SyncEngine::for_profile_dir` loads / generates the persistent
device identity under `<profile_data>/sync/device.json` and reads
the bearer token from `<profile_data>/sync/bearer.token`.
- `install_bearer` accepts (or clears) the Better Auth session
token; everything else stays inert until a token is on disk.
- `upload_now(&BrowserCore)` serialises the user's bookmarks into a
stable JSON snapshot, ships it via `SyncApiClient::upload_snapshot`,
and remembers the resulting snapshot id / logical clock / device
for the UI to surface.
- `BrowserCore::visible_bookmarks_for_sync` returns a read-only view
the engine can iterate without touching the in-memory state.
The shell / settings-page wiring that calls `upload_now` ships
separately so this commit stays a pure model-layer change with no
runtime behaviour difference until the UI plugs in.
ZacharyZhang-NY