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/Detail/PRDetailViewModel.swift

@@ -130,8 +130,16 @@ final class PRDetailViewModel: ObservableObject {
         busyMessage = "Merging…"
         defer { busyMessage = nil }
         do {
+            let deleteBranch = SettingsStore.shared.deleteBranchAfterMerge
             try await client.merge(owner: owner, repo: repo.name,
-                                   number: prNumber, method: method)
+                                   number: prNumber, method: method,
+                                   deleteBranchAfterMerge: deleteBranch)
+            // If the setting is on, delete the head branch after a successful
+            // merge (best-effort; the branch may already be gone or protected).
+            if deleteBranch, let head = pr?.head?.ref {
+                try? await client.deleteBranch(owner: owner, repo: repo.name,
+                                               branch: head)
+            }
             // Refresh the PR state to reflect the merge.
             pr = try? await client.pullRequest(owner: owner, repo: repo.name,
                                                number: prNumber)