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.App/ViewModels/VersionRowViewModel.cs

@@ -10,6 +10,7 @@ public enum VersionRowState
     InstalledIdle,        // Installée localement, pas d'action en cours
     AvailableIdle,        // Disponible côté serveur, pas installée
     Downloading,
+    Verifying,            // SHA-256 du ZIP téléchargé en cours
     Installing,
     Uninstalling,
 }
@@ -77,13 +78,17 @@ public sealed partial class VersionRowViewModel : ObservableObject
 
     public bool IsInstalled => State is VersionRowState.InstalledIdle or VersionRowState.Uninstalling;
     public bool IsRemoteOnly => State is VersionRowState.AvailableIdle;
-    public bool IsBusy => State is VersionRowState.Downloading or VersionRowState.Installing or VersionRowState.Uninstalling;
+    public bool IsBusy => State is VersionRowState.Downloading
+                                or VersionRowState.Verifying
+                                or VersionRowState.Installing
+                                or VersionRowState.Uninstalling;
 
     public string StateLabel => State switch
     {
         VersionRowState.InstalledIdle => Strings.StatusInstalled,
         VersionRowState.AvailableIdle => Strings.StatusAvailable,
         VersionRowState.Downloading   => Strings.StatusDownloading,
+        VersionRowState.Verifying     => Strings.StatusVerifying,
         VersionRowState.Installing    => Strings.StatusInstalling,
         VersionRowState.Uninstalling  => Strings.StatusUninstalling,
         _ => string.Empty