When a PR's head branch is behind main (or conflicts), Gitea's merge returns
409. Previously the app just showed an error. Now: on a 409 the app calls
POST /pulls/{index}/update (merges base into head), then retries the merge
once. Add APIError.mergeConflict for a clear message and map 409 in the
client. Bump to 2.6.
Multi-repo PR inbox, review (approve/request changes/comment), merge with
gating toggle, read-only diff viewer, and Gitea Actions check status with
one-tap re-run. Ships unsigned via the iMac runner to the SideStore source.