Merge icon, delete-branch-after-merge setting, floating tab bar; bump to 2.0

- Merge button: use the proper SF Symbol 'arrow.triangle.merge' instead of
  the download icon 'arrow.down.circle'.
- Add a Settings toggle 'Delete branch after merge'. When on, the merge call
  passes delete_branch_after_merge=true and the head branch is deleted after
  a successful merge (best-effort).
- PR detail bottom tab bar: floating capsule style (regularMaterial + shadow)
  like the latest iOS tab bars, instead of a full-width bar.
- Bump to 2.0.

GiteaPR/Settings/SettingsStore.swift

@@ -23,12 +23,17 @@ final class SettingsStore: ObservableObject {
         didSet { UserDefaults.standard.set(mergeGateMode.rawValue,
                                            forKey: "mergeGateMode") }
     }
+    @Published var deleteBranchAfterMerge: Bool {
+        didSet { UserDefaults.standard.set(deleteBranchAfterMerge,
+                                           forKey: "deleteBranchAfterMerge") }
+    }
 
     private init() {
         instanceURL = KeychainHelper.get("instanceURL") ?? "https://gitea.bueso.eu"
         token = KeychainHelper.get("token") ?? ""
         let raw = UserDefaults.standard.string(forKey: "mergeGateMode")
         mergeGateMode = MergeGateMode(rawValue: raw ?? "") ?? .warn
+        deleteBranchAfterMerge = UserDefaults.standard.bool(forKey: "deleteBranchAfterMerge")
     }
 
     var isConfigured: Bool {