Downloads: parallel multi-segment, sparse pre-alloc, faster SHA-256, URL auto-refresh

DownloadManager
- Parallel multi-segment download (default 8 connections, configurable up to 16)
  with per-segment Range requests. Bypasses OVH/Apache per-connection bandwidth
  throttling — typical speed-up ×4 to ×8 vs single connection.
- Reporter task on a dedicated Task with Interlocked aggregate counter (no lock
  contention with workers). Reports speed/ETA every 250ms even mid-download,
  fixes the "speed only shows at install transition" bug.
- Sparse file pre-allocation via FSCTL_SET_SPARSE before SetLength: no zero-fill,
  no SeManageVolumePrivilege, instant on any disk type. Removes 5-30s of
  preparation lag on HDD.
- HEAD probe skipped (trust manifest size, signed Ed25519). Falls back to
  single-segment if first segment returns 200 instead of 206.
- Resume URL comparison fixed: ignores HMAC querystring (?exp=&sig=) which
  changes per request, compares only host+path. Previously every resume started
  fresh because the old URL never matched the freshly signed one.
- Auto-refresh signed URL on 403/410 mid-DL: SemaphoreSlim with 5s debounce so
  8 simultaneous segment expirations trigger a single /api/download-url/ call.
  Slow-connection users (1 Mbps, 30+ hours for 14 GB) keep downloading
  transparently across multiple TTL cycles.
- Per-version hashAlgorithm:none in manifest skips client SHA-256 verification
  (still relying on Ed25519 manifest signature + HMAC URL).
- DangerButton style (red) added to Theme.xaml for the new cancel-resume action.

IntegrityService
- 16 MiB buffer (was 1 MiB), FileOptions.SequentialScan + Asynchronous,
  IncrementalHash (uses SHA-NI hardware extensions on .NET 8), double-buffering
  to overlap CPU and I/O. Typical 14 GB hash verification: 60-180s → 15-40s.

HttpClient
- MaxConnectionsPerServer=16, EnableMultipleHttp2Connections, HTTP/2 preferred,
  AutomaticDecompression=None (ZIPs are already compressed), 5min pooled
  connection lifetime.

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

src/PSLauncher.App/App.xaml.cs

@@ -108,13 +108,25 @@ public partial class App : Application
 
                 services.AddSingleton(sp =>
                 {
-                    var http = new HttpClient(new SocketsHttpHandler
+                    // Handler tuné pour les gros téléchargements parallèles :
+                    // - MaxConnectionsPerServer=16 pour autoriser le multi-segment (8 par défaut).
+                    // - AutomaticDecompression sur l'API JSON uniquement (None ici car les builds
+                    //   sont des ZIPs déjà compressés ; éviter la CPU+RAM gaspillée par un éventuel
+                    //   wrapping gzip côté serveur).
+                    // - PooledConnectionLifetime court pour éviter qu'OVH ferme nos sockets sous nous.
+                    var handler = new SocketsHttpHandler
                     {
                         PooledConnectionLifetime = TimeSpan.FromMinutes(5),
-                        AutomaticDecompression = System.Net.DecompressionMethods.All
-                    })
+                        PooledConnectionIdleTimeout = TimeSpan.FromSeconds(60),
+                        MaxConnectionsPerServer = 16,
+                        EnableMultipleHttp2Connections = true,
+                        AutomaticDecompression = System.Net.DecompressionMethods.None,
+                    };
+                    var http = new HttpClient(handler)
                     {
-                        Timeout = Timeout.InfiniteTimeSpan
+                        Timeout = Timeout.InfiniteTimeSpan,
+                        DefaultRequestVersion = System.Net.HttpVersion.Version20,
+                        DefaultVersionPolicy = HttpVersionPolicy.RequestVersionOrLower,
                     };
                     http.DefaultRequestHeaders.UserAgent.Add(new ProductInfoHeaderValue("PSLauncher", "0.5"));
                     return http;