Extract BranchesHelper.PostFetchRefresh to unify the two fetch paths

The post-fetch logic was duplicated in `backgroundFetch` and the manual
fetch handler: refresh a fixed set of views, then auto-forward branches
if the fetch succeeded. The two had already drifted on the refresh
scope; folding them into a single helper makes the duplication go
away and prevents it from drifting again.

Pass the fetch error through so we preserve the previous behaviour of
refreshing unconditionally but only auto-forwarding on success.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>