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/APIError.swift

@@ -7,6 +7,7 @@ enum APIError: LocalizedError, Equatable {
     case decoding(String)
     case unauthorized
     case notFound
+    case mergeConflict
 
     var errorDescription: String? {
         switch self {
@@ -18,6 +19,9 @@ enum APIError: LocalizedError, Equatable {
             if code == 401 || code == 403 {
                 return "Authentication failed. Check your token."
             }
+            if code == 409 {
+                return "This branch is out of date or conflicts with the base branch. Update the branch and try again."
+            }
             return message?.isEmpty == false
                 ? "Server error (\(code)): \(message!)"
                 : "Server error (\(code))."
@@ -27,6 +31,8 @@ enum APIError: LocalizedError, Equatable {
             return "Authentication failed. Check your token."
         case .notFound:
             return "Not found."
+        case .mergeConflict:
+            return "This branch is out of date or conflicts with the base branch. Update the branch and try again."
         }
     }
 }