feat: saveToken plugin for app groups on iOS

This will allow the share extension to access the Bearer token to send
images to the backend.

tauri-plugin-ios-shared-token/ios/Sources/SharedToken.swift

@@ -0,0 +1,34 @@
+import SwiftRs
+import Tauri
+import UIKit
+import WebKit
+
+class SaveTokenArgs: Decodable {
+    var token: String?
+}
+
+let appGroupID = "group.com.haystack.com"
+let sharedTokenKey = "sharedAuthToken"
+
+class SharedToken: Plugin {
+    @objc func saveToken(_ invoke: Invoke) throws {
+        guard let args = try? invoke.parseArgs(SaveTokenArgs.self), let token = args.token else {
+            invoke.reject("Missing token argument")
+            return
+        }
+
+        // Access shared UserDefaults
+        guard let sharedDefaults = UserDefaults(suiteName: appGroupID) else {
+            invoke.reject("Could not access shared UserDefaults. Check App Group configuration.")
+            return
+        }
+
+        sharedDefaults.set(token, forKey: sharedTokenKey)
+        invoke.resolve()
+    }
+}
+
+@_cdecl("init_plugin_ios_shared_token")
+func initPlugin() -> Plugin {
+  return SharedToken()
+}