fix(app): mark ImageCRCApp @mainactor for Swift 5.10 compatibility

CI on Swift 5.10 (Xcode 15.4) reported `call to main actor-isolated
initializer 'init(converter:fileChooser:)' in a synchronous nonisolated
context` at the @State default-value site for ConversionViewModel().
ConversionViewModel is @mainactor, so its initialiser is too.

Swift 6's SDK propagates @mainactor through SwiftUI's App protocol
implicitly; Swift 5.10's does not. Annotate explicitly so both
toolchains compile clean. Same pattern as the previous ContentView fix.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: spooosh

App/ImageCRCApp.swift

@@ -1,6 +1,7 @@
 import SwiftUI
 
 @main
+@MainActor
 struct ImageCRCApp: App {
     @State private var viewModel = ConversionViewModel()
     @AppStorage("appAppearance") private var appearance: AppAppearance = .system