Auto-update branch when merge conflicts (branch behind main); bump to 2.6

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.

GiteaPR/Networking/GiteaClient.swift

@@ -93,6 +93,9 @@ struct GiteaClient {
         case 404:
             await LogStore.shared.log(.error, "\(method) \(path) → 404 not found")
             throw APIError.notFound
+        case 409:
+            await LogStore.shared.log(.warn, "\(method) \(path) → 409 conflict")
+            throw APIError.mergeConflict
         default:
             let message = (try? Self.decoder.decode([String: String].self,
                                                      from: data))?["message"]
@@ -230,6 +233,14 @@ struct GiteaClient {
                               method: "DELETE")
     }
 
+    /// Merge the base branch into the head branch (`POST /pulls/{index}/update`).
+    /// When a PR is behind `main` (e.g. conflicts), the merge returns 409; this
+    /// rebases/syncs the head onto the base so it can then be merged.
+    func updateBranch(owner: String, repo: String, number: Int) async throws {
+        _ = try await request("repos/\(owner)/\(repo)/pulls/\(number)/update",
+                              method: "POST")
+    }
+
     func setState(owner: String, repo: String, number: Int,
                   state: String) async throws {
         struct Body: Encodable { let state: String }