feat(ios): a sideloadable iOS client over the shared Rust engine

A SwiftUI app that drives the same UniFFI engine as Android: Rust owns the
Proton sessions, SQLite and every decrypted byte, and Swift asks it questions
off the main thread. Playback goes through MPVKit's libmpv, with the pstr://
stream protocol ported from the Android JNI half, and secrets live in the
Keychain.

scripts/build-ios.sh builds the static library for aarch64-apple-ios, writes
the Swift bindings (module PstrBridge), generates the Xcode project with
XcodeGen and packages an unsigned .ipa for SideStore or AltStore, which
re-sign it with the viewer's own Apple ID. The generated project, Info.plist
and build output are ignored.

ios/ProtonStream/App/ProtonStreamApp.swift

@@ -0,0 +1,49 @@
+import AVFoundation
+import SwiftUI
+
+@main
+struct ProtonStreamApp: App {
+    @State private var model = AppModel()
+
+    init() {
+        // `.playback` keeps the audio going with the screen locked or the app
+        // in the background, which `UIBackgroundModes: audio` then permits.
+        try? AVAudioSession.sharedInstance().setCategory(.playback, mode: .moviePlayback)
+    }
+
+    var body: some Scene {
+        WindowGroup {
+            RootView()
+                .environment(model)
+                .preferredColorScheme(.dark)
+        }
+    }
+}
+
+struct RootView: View {
+    @Environment(AppModel.self) private var model
+
+    var body: some View {
+        if let failure = model.startupError {
+            ContentUnavailableView("Could not start", systemImage: "exclamationmark.triangle", description: Text(failure))
+        } else {
+            TabView {
+                NavigationStack { LibraryView() }
+                    .tabItem { Label("Library", systemImage: "play.rectangle.on.rectangle") }
+                NavigationStack { SharesView() }
+                    .tabItem { Label("Shares", systemImage: "link") }
+                NavigationStack { SettingsView() }
+                    .tabItem { Label("Settings", systemImage: "gearshape") }
+            }
+            .task { await model.reload() }
+            .alert("Something went wrong", isPresented: .init(
+                get: { model.error != nil },
+                set: { if !$0 { model.error = nil } }
+            )) {
+                Button("OK", role: .cancel) {}
+            } message: {
+                Text(model.error ?? "")
+            }
+        }
+    }
+}