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/Resources/Theme.xaml

@@ -135,6 +135,39 @@
         </Setter>
     </Style>
 
+    <!--
+        DangerButton : action destructive (rouge). Utilisé pour le bouton « Annuler »
+        à côté de « ↻ Reprendre » sur les rows ayant un téléchargement partiel — pour
+        que l'utilisateur puisse abandonner le partial et repartir de zéro.
+    -->
+    <Style x:Key="DangerButton" TargetType="Button" BasedOn="{StaticResource PrimaryButton}">
+        <Setter Property="Background" Value="#DC2626" />
+        <Setter Property="FontSize" Value="13" />
+        <Setter Property="FontWeight" Value="SemiBold" />
+        <Setter Property="Padding" Value="14,8" />
+        <Setter Property="Template">
+            <Setter.Value>
+                <ControlTemplate TargetType="Button">
+                    <Border x:Name="Bd"
+                            Background="{TemplateBinding Background}"
+                            CornerRadius="4"
+                            Padding="{TemplateBinding Padding}">
+                        <ContentPresenter HorizontalAlignment="Center" VerticalAlignment="Center" />
+                    </Border>
+                    <ControlTemplate.Triggers>
+                        <Trigger Property="IsMouseOver" Value="True">
+                            <Setter TargetName="Bd" Property="Background" Value="#EF4444" />
+                        </Trigger>
+                        <Trigger Property="IsEnabled" Value="False">
+                            <Setter TargetName="Bd" Property="Background" Value="#444" />
+                            <Setter Property="Foreground" Value="#888" />
+                        </Trigger>
+                    </ControlTemplate.Triggers>
+                </ControlTemplate>
+            </Setter.Value>
+        </Setter>
+    </Style>
+
     <!-- Boutons de chrome window (min / max / close), 46x40 façon Windows -->
     <Style x:Key="WindowControlButton" TargetType="Button">
         <Setter Property="Background" Value="Transparent" />