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/Detail/PRDetailViewModel.swift

@@ -144,6 +144,23 @@ final class PRDetailViewModel: ObservableObject {
             pr = try? await client.pullRequest(owner: owner, repo: repo.name,
                                                number: prNumber)
             return true
+        } catch APIError.mergeConflict {
+            // Branch is behind main (or conflicts). Update it and retry once.
+            busyMessage = "Updating branch…"
+            do {
+                try await client.updateBranch(owner: owner, repo: repo.name,
+                                              number: prNumber)
+                busyMessage = "Merging…"
+                try await client.merge(owner: owner, repo: repo.name,
+                                       number: prNumber, method: method,
+                                       deleteBranchAfterMerge: SettingsStore.shared.deleteBranchAfterMerge)
+                pr = try? await client.pullRequest(owner: owner, repo: repo.name,
+                                                   number: prNumber)
+                return true
+            } catch {
+                errorMessage = error.localizedDescription
+                return false
+            }
         } catch {
             errorMessage = error.localizedDescription
             return false