Add iOS app with CarPlay and Siri

SwiftUI port of the Android app under ios/, with CarPlay browse and
Now Playing, Siri play-media intents, and a build-ipa.sh script that
makes an unsigned IPA for sideloading. README gets an iOS section and
the vibecoded disclaimer.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

ios/Stemify/UI/Nav.swift

@@ -0,0 +1,65 @@
+import SwiftUI
+
+enum Tab: String, CaseIterable, Hashable {
+    case Library, Playlists, Search, Downloads, Settings
+
+    var icon: String {
+        switch self {
+        case .Library: "square.stack.fill"
+        case .Playlists: "music.note.list"
+        case .Search: "magnifyingglass"
+        case .Downloads: "arrow.down.circle.fill"
+        case .Settings: "gearshape.fill"
+        }
+    }
+}
+
+enum Route: Hashable {
+    case Artists, Albums, Songs, Playlists
+    case Eq, EqFilters(profileId: String), EqFilterPage(profileId: String, filterId: String)
+    case PlexAccount, StemServer, Quality, Storage, Appearance
+    case ArtistPage(Artist), AlbumPage(Album), PlaylistPage(Playlist)
+}
+
+/// Tabs, their stacks and the Now Playing sheet: the things any screen can ask of the shell.
+@MainActor
+@Observable
+final class Ui {
+    var tab: Tab = .Library
+    var paths: [Tab: [Route]] = [:]
+    var nowPlayingOpen = false {
+        didSet { overlays.level = nowPlayingOpen ? 1 : 0 }
+    }
+    let overlays: Overlays
+
+    init(overlays: Overlays) { self.overlays = overlays }
+
+    func path(_ t: Tab) -> Binding<[Route]> {
+        Binding(get: { self.paths[t] ?? [] }, set: { self.paths[t] = $0 })
+    }
+
+    func push(_ r: Route) { paths[tab, default: []].append(r) }
+
+    func pop() { if paths[tab]?.isEmpty == false { paths[tab]?.removeLast() } }
+
+    /// Tapping the selected tab again pops it to its root.
+    func selectTab(_ t: Tab) {
+        if t == tab { paths[t] = [] } else { tab = t }
+    }
+
+    func openNowPlaying() { nowPlayingOpen = true }
+
+    /// Switches to the Library tab and pushes [r] there.
+    func showInLibrary(_ r: Route) {
+        nowPlayingOpen = false
+        tab = .Library
+        paths[.Library, default: []].append(r)
+    }
+
+    /// Switches to Settings and opens [r] (the Equalizer) there.
+    func showInSettings(_ r: Route) {
+        nowPlayingOpen = false
+        tab = .Settings
+        if paths[.Settings]?.last != r { paths[.Settings, default: []].append(r) }
+    }
+}