v0.20.0 — Cancel-from-red-button waits for segments + new Verifying row state

Two related UX bugs fixed.

1) Red "Annuler" button on a row : DL appeared to keep going

The button calls RestartFromZeroAsync which used to fire
_activeDownloadCts.Cancel() then sleep 200 ms then delete the .partial.
With 16 parallel segments mid-write, 200 ms is way too short — the
segments were still flushing buffers when the file got deleted, then
recreated it from their own write streams, making it look like the
download "continued".

Now we keep a reference to the in-flight install Task
(_activeInstallTask) and properly await it (with a 10 s safety timeout)
before discarding state. This guarantees all segment FileStreams are
closed and the .partial deletion is the final word.

2) Progress bar said "Downloading…" during SHA-256 verification

The footer message switched to "🔍 Vérification SHA-256 v…" but the
row's badge stayed on "⬇ Downloading…" because row.State stayed at
Downloading throughout DownloadAsync (which internally chains DL +
verify). New VersionRowState.Verifying inserted between Downloading
and Installing, applied on the first hashProgress callback. Badge now
reads "🔍 Vérification…" / "🔍 Verifying…" during the hash phase.

VersionRowViewModel.IsBusy now also includes Verifying so commands
that gate on busy stay disabled during verification.

Versions bumped to 0.20.0.

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

src/PSLauncher.Core/Localization/Strings.cs

@@ -95,6 +95,7 @@ public static class Strings
     public static string StatusDownloading  => T("⬇ Téléchargement…",    "⬇ Downloading…",       "⬇ 下载中…",    "⬇ กำลังดาวน์โหลด…",  "⬇ جارٍ التنزيل…");
     public static string StatusInstalling   => T("📦 Installation…",     "📦 Installing…",       "📦 安装中…",   "📦 กำลังติดตั้ง…",    "📦 جارٍ التثبيت…");
     public static string StatusUninstalling => T("🗑 Suppression…",      "🗑 Uninstalling…",     "🗑 卸载中…",   "🗑 กำลังถอนการติดตั้ง…", "🗑 جارٍ الإزالة…");
+    public static string StatusVerifying    => T("🔍 Vérification…",     "🔍 Verifying…",        "🔍 校验中…",   "🔍 กำลังตรวจสอบ…",     "🔍 جارٍ التحقق…");
 
     // ==================== ACTIONS ====================
     public static string ActionLaunch       => T("▶  Lancer",       "▶  Launch",       "▶  启动", "▶  เปิด",     "▶  تشغيل");