[dialog] [iOS] Dialog opens on most recently created window instead of the calling window (multi-scene apps)

### Describe the bug

In a multi-window iOS app with multiple scene (`UIScene`) support enabled, dialogs opened via the dialog plugin are always presented on the most recently created window, regardless of which window invoked the command.

If window A is open, then window B is created, and the user triggers a dialog from window A, the dialog appears over window B instead. When B is the frontmost/key scene, dialogs meant for A show up in the wrong scene and are effectively unusable.

### Reproduction

Steps to reproduce:
1. Enable iOS multi-window (UIScene) support and handle `RunEvent::SceneRequested`
2. Create additional windows (per the Tauri mobile multi-window guide).
3. Open the app on iPad so two windows/scenes are visible. From the first window, call any dialog API:

```ts
import { message } from '@tauri-apps/plugin-dialog'
await message('Hello from window A', { title: 'Window A' })
```

4. Observe that the dialog is presented over the second (most recently created) window rather than window A.

### Expected behavior

The dialog should be presented from the window/scene that invoked the command. The plugin command already carries the originating webview label, so presentation should be anchored to that window's `UIWindowScene` / root view controller rather than the current key window or foreground scene.

### Full `tauri info` output

```text
[✔] Environment
- OS: Mac OS 26.5.2 arm64 (X64)
✔ Xcode Command Line Tools: installed
✔ Xcode: 26.6
✔ rustc: 1.94.0 (4a4ef493e 2026-03-02)
✔ cargo: 1.94.0 (85eff7c80 2026-01-15)
✔ rustup: 1.28.2 (e4f3ad6f8 2025-04-28)
✔ Rust toolchain: 1.94.0-aarch64-apple-darwin
- node: 24.13.1
- npm: 11.8.0
- bun: 1.3.12

[-] Packages
- tauri 🦀: 2.11.3, (outdated, latest: 2.11.5)
- tauri-build 🦀: 2.6.3
- wry 🦀: 0.55.1
- tao 🦀: 0.35.3
- tauri-cli 🦀: 2.9.6 (outdated, latest: 2.11.4)
- @tauri-apps/api ⱼₛ: 2.11.0 (outdated, latest: 2.11.1)
- @tauri-apps/cli ⱼₛ: 2.11.0 (outdated, latest: 2.11.4)

[-] Plugins
- tauri-plugin-os 🦀: 2.3.2
- @tauri-apps/plugin-os ⱼₛ: 2.3.2
- tauri-plugin-dialog 🦀: 2.7.1
- @tauri-apps/plugin-dialog ⱼₛ: 2.7.0 (outdated, latest: 2.7.1)
- tauri-plugin-opener 🦀: 2.5.4
- @tauri-apps/plugin-opener ⱼₛ: 2.5.3 (outdated, latest: 2.5.4)
- tauri-plugin-store 🦀: 2.4.3
- @tauri-apps/plugin-store ⱼₛ: 2.4.2 (outdated, latest: 2.4.3)
- tauri-plugin-log 🦀: 2.8.0, (outdated, latest: 2.9.0)
- @tauri-apps/plugin-log ⱼₛ: 2.8.0 (outdated, latest: 2.9.0)
- tauri-plugin-fs 🦀: 2.5.1
- @tauri-apps/plugin-fs ⱼₛ: 2.5.0 (outdated, latest: 2.5.1)
```

### Additional context

_No response_

Start by tracing the dialog plugin command from its TypeScript API through the originating webview label to iOS presentation. Reproduce the issue with UIScene support, multiple windows, and RunEvent::SceneRequested as described. Done means a dialog invoked from window A is presented from A's UIWindowScene/root view controller rather than the most recently created or key window.

Written by the indexing model from the issue text.