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/SettingsView.swift

@@ -5,6 +5,7 @@ struct SettingsView: View {
     @State private var instanceURL: String
     @State private var token: String
     @State private var mergeGateMode: MergeGateMode
+    @State private var deleteBranchAfterMerge: Bool
     @State private var testResult: TestResult?
     @State private var testing = false
     @State private var showLogoutConfirm = false
@@ -18,6 +19,7 @@ struct SettingsView: View {
         _instanceURL = State(initialValue: SettingsStore.shared.instanceURL)
         _token = State(initialValue: SettingsStore.shared.token)
         _mergeGateMode = State(initialValue: SettingsStore.shared.mergeGateMode)
+        _deleteBranchAfterMerge = State(initialValue: SettingsStore.shared.deleteBranchAfterMerge)
     }
 
     var body: some View {
@@ -49,6 +51,7 @@ struct SettingsView: View {
                     Text("Warn mode shows CI status and allows merging a red PR after confirmation. Block mode disables merge while any check is failing.")
                         .font(.caption)
                         .foregroundColor(.secondary)
+                    Toggle("Delete branch after merge", isOn: $deleteBranchAfterMerge)
                 }
 
                 Section("Diagnostics") {
@@ -130,6 +133,7 @@ struct SettingsView: View {
             instanceURL.trimmingCharacters(in: .whitespacesAndNewlines)
         SettingsStore.shared.token = token
         SettingsStore.shared.mergeGateMode = mergeGateMode
+        SettingsStore.shared.deleteBranchAfterMerge = deleteBranchAfterMerge
         dismiss()
     }
 }