Summary

Follow-up to #111 as suggested by @pol-rivero.

After PR #111 added a fetch before the merge, performFetch also fast-forwards local branches as a side effect.
By merging origin/main directly (instead of local main) and skipping the fast-forward step, we avoid unintended changes to local branches.

So, instead of merging the local default branch (e.g. main), with this PR it merges the remote tracking branch (e.g. origin/main) directly. This means the "Update from Default Branch" action no longer alters local branches as a side effect. The menu label now reflects this (e.g. "Update from origin/main").

Changes

app/src/ui/app.tsx

• Made updateBranchWithContributionTargetBranch() async so it can await the fetch before merging.
• After fetching, resolves the remote tracking branch (e.g. origin/main) from allBranches and merges that instead of the local branch. Falls back to the local branch if no remote tracking branch is found.
• Added getRemoteTrackingBranch() helper: for remote branches (fork case) returns the branch as-is; for local branches, looks up the corresponding remote branch via the upstream field.

app/src/ui/dispatcher/dispatcher.ts

• Added fetchWithoutFastForward(): exposes the new app-store method to the UI layer.

app/src/lib/stores/app-store.ts

• Added skipFastForward parameter to performFetch(). When true, skips the fastForwardBranches() call so local branches are not altered.
• Added _fetchWithoutFastForward() public method that calls performFetch with skipFastForward: true.
• Updated the menu label source to use the branch's upstream name (e.g. origin/main) instead of the local name (e.g. main), so the menu reads "Update from origin/main".

Test plan

• Click Branch > Update from origin/main when the remote default branch has unpulled commits. Confirm the fetch runs and the merge succeeds.
• Click Branch > Update from origin/main when the branch is genuinely up to date. Confirm the app correctly reports "already up to date".
• Verify the local main branch is NOT fast-forwarded after the operation.
• Verify the menu label shows "Update from origin/main" (or the appropriate remote branch name).
• For fork repos contributing to parent: verify the menu shows "Update from upstream/main" and merges the upstream remote branch.