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>

installer/PSLauncher.iss

@@ -11,7 +11,7 @@
 
 #define MyAppName        "PROSERVE Launcher"
 #define MyAppShortName   "PS_Launcher"
-#define MyAppVersion     "0.19.0"
+#define MyAppVersion     "0.20.0"
 #define MyAppPublisher   "ASTERION VR"
 #define MyAppURL         "https://asterionvr.com"
 #define MyAppExeName     "PS_Launcher.exe"