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.

scripts/render-ios-icon.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+# Render ios/ProtonStream/Resources/AppIcon.svg into the asset catalog's one
+# 1024 px icon. iOS wants it opaque, so the alpha channel is dropped.
+set -euo pipefail
+
+script_dir="$(cd -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd)"
+resources="$script_dir/../ios/ProtonStream/Resources"
+out="$resources/Assets.xcassets/AppIcon.appiconset/AppIcon-1024.png"
+
+command -v rsvg-convert >/dev/null || { echo "rsvg-convert is required (librsvg)" >&2; exit 1; }
+command -v magick >/dev/null || { echo "magick is required (ImageMagick)" >&2; exit 1; }
+rsvg-convert -w 1024 -h 1024 "$resources/AppIcon.svg" | magick - -alpha off -strip "$out"
+echo "wrote $out"