feat(ios): port the Android client to SwiftUI

The iOS client is now a screen-for-screen port of the Android app over the
same pstr-android UniFFI surface, with no Rust changes: Library, title page,
History, Shares (public links and Drive folders), Downloads and Settings,
with the same strings, undo snackbars and two-pane layout on wide screens.

- Theme from the engine's palette (Material role names, Inter, the same
  type ramp and corners), repainted live from the appearance settings.
- Account sign-in with two-factor code, mailbox password and the Proton
  CAPTCHA in a non-persistent WKWebView, the Drive folder browser, and
  watch-history sync every five minutes, after sign-in, on player close and
  when the app leaves the screen.
- Player over libmpv: resume, progress saved every 5 s, autoplay with the
  up-next card, skip and auto-skip from the chapter plan, track, speed,
  volume and episode pickers, landscape lock, mini transport, and
  lock-screen controls with interruption and route-change handling.
- Offline downloads with the Android record format, free-space floor,
  retries, transfer rate and the Wi-Fi-only policy; interrupted transfers
  resume at next launch.
- Config, data and download state are excluded from backup, as Android's
  allowBackup=false: a restore would bring shares back without their
  Keychain secrets and give two devices one sync identity.

Picture-in-picture, the share sheet and background downloads are not
possible the same way on iOS; docs/IOS.md lists why and what replaces them.

Adds XCTest ports of the Android unit tests, a `test` command to
scripts/build-ios.sh, the app icon and its render script, and an iOS
workflow on macos-15 that runs the tests and builds the unsigned .ipa.

ios/ProtonStream/UI/DownloadsView.swift

@@ -0,0 +1,162 @@
+import SwiftUI
+
+/// One show's saved episodes, and the show itself where the library has it.
+private struct OfflineGroup: Identifiable {
+    let name: String
+    let title: TitleRecord?
+    let files: [OfflineRecord]
+    var id: String { name }
+}
+
+struct DownloadsView: View {
+    @Environment(AppModel.self) private var model
+    @Environment(\.scheme) private var scheme
+    // Deleting a saved episode asks first: it is the viewer's own choice to
+    // keep it, and getting it back is a download, not a tap.
+    @State private var deleting: OfflineRecord?
+
+    var body: some View {
+        let retained = DownloadCoordinator.shared.records
+        let offline = model.offline
+        TabPage("Downloads") {
+            if retained.isEmpty && offline.isEmpty {
+                EmptyState("No downloads", "Episodes, seasons and shows saved offline appear here.")
+            } else {
+                ScrollView {
+                    LazyVStack(alignment: .leading, spacing: 0) {
+                        if !offline.isEmpty {
+                            Text("\(offline.count) \(offline.count == 1 ? "episode" : "episodes") · \(formatBytes(offline.reduce(0) { $0 + $1.size })) on this device")
+                                .textStyle(.bodyMedium)
+                                .foregroundStyle(scheme.onSurfaceVariant)
+                                .padding(.horizontal, 16)
+                                .padding(.vertical, 4)
+                        }
+                        // What is still arriving comes first: it is the part of
+                        // the page that changes, and the part with something to do.
+                        if !retained.isEmpty {
+                            SectionHeading(title: "In progress")
+                            ForEach(retained) { download in
+                                PartialRow(download: download)
+                            }
+                        }
+                        // Under the show they belong to, as on desktop: a saved
+                        // season is fourteen rows, and fourteen rows of
+                        // "Episode 3" name nothing.
+                        ForEach(groups(offline)) { group in
+                            SectionHeading(
+                                title: group.name,
+                                caption: "\(group.files.count) \(group.files.count == 1 ? "episode" : "episodes") · \(formatBytes(group.files.reduce(0) { $0 + $1.size }))"
+                            )
+                            ForEach(group.files, id: \.linkId) { file in
+                                SavedRow(file: file, title: group.title) { deleting = file }
+                            }
+                        }
+                    }
+                    .padding(.bottom, 24)
+                }
+            }
+        }
+        .alert(
+            "Delete this download?",
+            isPresented: Binding(get: { deleting != nil }, set: { if !$0 { deleting = nil } }),
+            presenting: deleting
+        ) { file in
+            Button("Delete", role: .destructive) { model.removeOffline(file) }
+            Button("Cancel", role: .cancel) {}
+        } message: { file in
+            Text("\(file.episode?.label ?? "The episode") (\(formatBytes(file.size))) leaves this device. It still streams, and can be downloaded again.")
+        }
+    }
+
+    /// Which show each saved file is from. The offline record knows its
+    /// episode but not its show, and the library is the only thing that does.
+    private func groups(_ offline: [OfflineRecord]) -> [OfflineGroup] {
+        var shows: [String: TitleRecord] = [:]
+        for title in model.titles {
+            for episode in title.playlist { shows["\(episode.shareId)/\(episode.linkId)"] = title }
+        }
+        let grouped = Dictionary(grouping: offline) { shows["\($0.shareId)/\($0.linkId)"]?.key }
+        return grouped.map { key, files in
+            let title = key.flatMap { key in model.titles.first { $0.key == key } }
+            return OfflineGroup(name: title?.displayName ?? "Not in the library", title: title, files: files)
+        }
+        .sorted { $0.name < $1.name }
+    }
+}
+
+private struct SectionHeading: View {
+    let title: String
+    var caption: String?
+    @Environment(\.scheme) private var scheme
+
+    var body: some View {
+        VStack(alignment: .leading, spacing: 0) {
+            Text(title).textStyle(.titleMedium).foregroundStyle(scheme.onSurface).lineLimit(1)
+            if let caption { Text(caption).textStyle(.bodySmall).foregroundStyle(scheme.onSurfaceVariant) }
+        }
+        .padding(EdgeInsets(top: 20, leading: 16, bottom: 4, trailing: 16))
+    }
+}
+
+/// A saved episode: its still, what it is, its size, and a way to remove it.
+private struct SavedRow: View {
+    let file: OfflineRecord
+    let title: TitleRecord?
+    let onDelete: () -> Void
+    @Environment(\.scheme) private var scheme
+
+    var body: some View {
+        let episode = file.episode
+        HStack(spacing: 16) {
+            RemoteArtwork(episode?.stillUrl ?? title?.backdropUrl, title?.name ?? file.linkId, fallback: episode?.thumbnailSource, labelled: episode == nil)
+                .frame(width: 96)
+                .aspectRatio(16 / 9, contentMode: .fit)
+                .clipShape(RoundedRectangle(cornerRadius: Corner.standard))
+            VStack(alignment: .leading, spacing: 2) {
+                Text(episode?.heading(0) ?? file.linkId).textStyle(.bodyLarge).foregroundStyle(scheme.onSurface).lineLimit(2)
+                Text([episode?.label, formatBytes(file.size)].compactMap { $0 }.joined(separator: " · "))
+                    .textStyle(.bodyMedium).foregroundStyle(scheme.onSurfaceVariant)
+            }
+            Spacer(minLength: 0)
+            IconButton("trash", "Delete download", action: onDelete)
+        }
+        .padding(.leading, 16)
+        .padding(.trailing, 4)
+        .padding(.vertical, 8)
+    }
+}
+
+/// A download not yet finished: how far it has got, why it stopped if it did,
+/// and pause or resume beside cancel.
+private struct PartialRow: View {
+    let download: RetainedDownload
+    @Environment(AppModel.self) private var model
+    @Environment(\.scheme) private var scheme
+
+    var body: some View {
+        let active = download.status == RetainedDownload.statusRunning || download.status == RetainedDownload.statusQueued
+        HStack(spacing: 8) {
+            VStack(alignment: .leading, spacing: 2) {
+                Text(download.label).textStyle(.bodyLarge).foregroundStyle(scheme.onSurface).lineLimit(2)
+                Text(downloadStatusLine(download)).textStyle(.bodyMedium).foregroundStyle(scheme.onSurfaceVariant)
+                if download.total > 0 {
+                    AccentProgress(progress: Double(download.downloaded) / Double(download.total)).padding(.top, 8)
+                }
+                if let error = download.error {
+                    Text(error).textStyle(.bodyMedium).foregroundStyle(scheme.error)
+                }
+            }
+            Spacer(minLength: 0)
+            if active {
+                IconButton("pause.fill", "Pause download") { model.pauseDownload(download) }
+            } else {
+                IconButton("play.fill", "Resume download") { model.resumeDownload(download) }
+            }
+            // What has arrived so far is discarded; nothing finished is.
+            IconButton("xmark", "Cancel and delete what has arrived") { model.deletePartial(download) }
+        }
+        .padding(.leading, 16)
+        .padding(.trailing, 4)
+        .padding(.vertical, 8)
+    }
+}