fix(mobile): show the Join prompt when an invite launches Buzz (#8970)

## Summary

Opening an invite while Buzz is closed can leave the Join prompt
invisible and block later attempts. Native confirmation can attach to a
temporary system dialog that then closes, leaving the join result
pending.

This fix uses Flutter confirmation when the native presenter is
occupied. If UIKit rejects presentation, it clears the pending session
so the user can try again.

### Related issue

Related invite handoff: https://github.com/block/buzz/pull/8601.

### Testing

The native regression fails before the fix and passes afterward. It
covers rejected presentation, a second attempt, and an existing
temporary dialog.

The isolated PR build passed a fresh-user journey from Safari consent
through launching Buzz, visible confirmation, Join, starter channels and
seeded history.

### Screenshots

Visible Join prompt after Safari opens Buzz, from this PR build:

![Join prompt after cold
launch](https://raw.githubusercontent.com/block/buzz/f4d083ffaa2b2d2d2672d2b5c9a5f9f3690d75b1/pr-8970--COLD_CONFIRMATION.png)

### Remaining work

Finish Dart validation and review before readiness.

Signed-off-by: John Tennant <jtennant@block.xyz>
Co-authored-by: Zack <04f47b290b155b3b8b782b11501cdb15f1e21e82dfceefd01ff0ed405459d927@buzz.block.builderlab.xyz>

mobile/ios/Runner/NativeCommunityEntry.swift

@@ -24,10 +24,9 @@ final class NativeCommunityEntryCoordinator: NSObject, UIAdaptivePresentationCon
       let root = UIApplication.shared.connectedScenes.compactMap { $0 as? UIWindowScene }
         .filter { $0.activationState == .foregroundActive }.flatMap(\.windows)
         .first(where: \.isKeyWindow)?.rootViewController
-      guard var presenter = root else {
+      guard let presenter = root, presenter.presentedViewController == nil else {
         result(FlutterError(code: "presentation_failed", message: nil, details: nil)); return
       }
-      while let presented = presenter.presentedViewController { presenter = presented }
       let controller = NativeCommunityEntryController(configuration: args)
       controller.action = { [weak self] method, payload, completion in
         guard let self, self.session == session else { return }
@@ -47,6 +46,10 @@ final class NativeCommunityEntryCoordinator: NSObject, UIAdaptivePresentationCon
       self.navigation = navigation
       controller.configureDetents()
       presenter.present(navigation, animated: !UIAccessibility.isReduceMotionEnabled)
+      guard presenter.presentedViewController === navigation else {
+        self.complete(FlutterError(code: "presentation_failed", message: nil, details: nil))
+        return
+      }
       navigation.presentationController?.delegate = self
     }
   }
@@ -71,12 +74,12 @@ final class NativeCommunityEntryCoordinator: NSObject, UIAdaptivePresentationCon
     }
   }
 
-  private func complete(_ joined: Bool) {
+  private func complete(_ value: Any?) {
     let result = pending
     pending = nil
     navigation = nil
     session = nil
-    result?(joined)
+    result?(value)
   }
 }
 
@@ -236,6 +239,7 @@ private final class NativeCommunityEntryController: UIViewController, UITextFiel
         submit = addButton("Join community", primary: true) { [weak self] in
           self?.request("join", [:])
         }
+        submit?.accessibilityIdentifier = "community-invite-join"
       } else {
         for choice in choices {
           addButton("\(choice["label"] ?? "Saved identity")\n\(choice["npub"] ?? "")") { [weak self] in

mobile/ios/RunnerTests/RunnerTests.swift

@@ -8,6 +8,39 @@ import XCTest
 
 class RunnerTests: XCTestCase {
 
+  @MainActor
+  func testRejectedCommunityEntryPresentationCompletesAndAllowsRetry() throws {
+    let messenger = NavigationTestMessenger()
+    let presenter = RejectedCommunityEntryPresenter()
+    let window = UIWindow(frame: UIScreen.main.bounds)
+    window.windowScene = UIApplication.shared.connectedScenes.compactMap { $0 as? UIWindowScene }.first
+    window.rootViewController = presenter
+    window.makeKeyAndVisible()
+    defer { window.isHidden = true }
+    let coordinator = NativeCommunityEntryCoordinator(messenger: messenger)
+    for session in [1, 2] {
+      var reply: Any?
+      messenger.invoke("present", arguments: ["session": session, "name": "Local test", "invite": true]) { reply = $0 }
+      XCTAssertEqual((reply as? FlutterError)?.code, "presentation_failed")
+      XCTAssertEqual(presenter.attempts, session)
+    }
+    let root = UIViewController()
+    window.rootViewController = root
+    window.makeKeyAndVisible()
+    let temporary = UIViewController()
+    let presented = expectation(description: "Temporary native modal appeared")
+    root.present(temporary, animated: false) { presented.fulfill() }
+    wait(for: [presented], timeout: 2)
+    XCTAssertTrue(root.presentedViewController === temporary)
+    XCTAssertNotNil(temporary.view.window)
+    var reply: Any?
+    messenger.invoke("present", arguments: ["session": 3, "name": "Local test", "invite": true]) { reply = $0 }
+    XCTAssertEqual((reply as? FlutterError)?.code, "presentation_failed")
+    XCTAssertNil(temporary.presentedViewController)
+    root.dismiss(animated: false)
+    withExtendedLifetime(coordinator) {}
+  }
+
   @MainActor
   func testCustomFlutterHeaderUsesDecorativeSoftEdge() throws {
     guard #available(iOS 27.0, *) else { return }
@@ -2085,3 +2118,11 @@ private final class NavigationTestMessenger: NSObject, FlutterBinaryMessenger {
     handler?(message) { _ in }
   }
 }
+
+private final class RejectedCommunityEntryPresenter: UIViewController {
+  var attempts = 0
+  override func present(_ viewControllerToPresent: UIViewController, animated flag: Bool, completion: (() -> Void)? = nil) {
+    // Reproduce UIKit declining a presentation during a scene transition.
+    attempts += 1
+  }
+}