feat: Add native iOS download support using URLSession

[velocitysystems]

This PR introduces native iOS download support using URLSession splitting the original shared code into platform-specific modules (desktop and mobile). It utilises a new Swift package DownloadManagerKit which is integrated with the Tauri plugin architecture.

Key Changes

• build.rs: Added ios_path("ios") to the tauri_plugin builder to include the iOS implementation
• guest-js/index.ts: Added addPluginListener to handle events from the iOS plugin
• ios-src/DownloadManagerKit: Created a Swift package containing the core download logic
• ios-src/DownloadManagerExample: Created an example iOS app that utilizes the DownloadManagerKit package.

Impact

• Behavioral changes: Introduces iOS support for the download plugin, using a native Swift implementation. The JS API now supports listening to plugin events from both desktop and mobile platforms.
• Dependencies affected: Adds a dependency on the new DownloadManagerKit Swift package for iOS builds, and Tauri package.
• Splits plugin implementation into platform-specific modules, requiring conditional compilation.

Maintenance

• Updated all Tauri packages to latest
• Updated generated iOS example project to fix arm64-sim bug with Xcode 16.3
• Updated package.json to add new run targets for iOS and Android

[jjhafer]

@velocitysystems Finished my initial review and offered suggestions. We can certainly discuss which suggestions are actually necessary before this can be merged.

[velocitysystems]

@jjhafer With async/await this could be simplified to:

   func getDownloadTask(_ key: String) async -> URLSessionDownloadTask? {
       guard let session = session else { return nil }
       let tasks = await withCheckedContinuation { continuation in
           session.getAllTasks { tasks in
               continuation.resume(returning: tasks)
           }
       }
       
       return tasks.compactMap { $0 as? URLSessionDownloadTask }.first { $0.taskDescription == key }
   }

However from testing I ran into issues with invoking the commands from Tauri's IPC when either:

• Decorated with async
• Using an embedded Task { } wrapper

Will do some further research and may revisit this in a future PR.

[velocitysystems]

This command allows the JS API to determine whether to register a plugin listener for iOS or a global listener for Android/desktop. Adding this command was a low-cost alternative to bringing in an external dependency such as the OS information plugin.

[yokuze]

Thank you for all of the great work here! A few questions/suggestions there for you.

[velocitysystems]

Thank you for all of the great work here! A few questions/suggestions there for you.

Thanks @yokuze. I've addressed all of your feedback in the PR.