FN-5877: add sync-with-origin remotes action

Add a one-click Remotes sync flow that rebases on pull before pushing.

- add a Sync button in Git Manager Remotes that runs pull --rebase and then push
- stop before pushing when the rebase pull reports a conflict or fails, and surface toasts/status updates
- cover successful, conflicting, failing, and loading sync behavior in GitManagerModal tests
- document the new Sync action in the dashboard guide

Files changed:
 docs/dashboard-guide.md                            |   1 +
 .../dashboard/app/components/GitManagerModal.tsx   |  39 ++++++++
 .../components/__tests__/GitManagerModal.test.tsx  | 100 +++++++++++++++++++++
 3 files changed, 140 insertions(+)

Fusion-Task-Id: FN-5877

Fusion-Task-Lineage: 2e28c00d-b965-4f44-a6fa-26536d91d16d

docs/dashboard-guide.md

@@ -239,6 +239,7 @@ Features:
 - Commit and diff browsing
 - Push/pull/fetch actions
 - Pull with rebase option (split-button chooses between `git pull` and `git pull --rebase`)
+- One-click **Sync** action in Remotes (`git pull --rebase` followed by push; it stops and surfaces an error instead of pushing when the pull conflicts or fails)
 - Remote editing controls
 - Stash inspection (view stat + patch) before apply/pop/drop actions
 - Remotes tab keeps "Recent commits on {remote}" in sync immediately after successful push/pull actions