v0.19.0 — Bump default parallel segments 8→16 to shorten end-of-DL tail

Issue : on a 14 GB download with 8 segments, when 7 finish there's only
1 connection left handling its assigned 1.75 GB → apparent throughput
collapses to 1/8th of the peak for the final stretch (~5 min visible
slowdown at the end).

Quick fix : double the default segment count.
- 8 segments  : last has 1.75 GB to finish alone
- 16 segments : last has 875 MB → tail ~halved

Tuning surfaced in Settings → Avancés → Serveur as
« Connexions parallèles pour les téléchargements (1-32, défaut 16) ».
Power users on faster servers can push to 24-32. OVH mutualisé tolerates
up to 16-24 ; above that, soft rate-limiting kicks in.

Aligned :
- LocalConfig default : 8 → 16
- DownloadManager clamp : Math.Clamp(_, 1, 16) → 1, 32
- HttpClient handler MaxConnectionsPerServer : 16 → 32 (matches new ceiling)

Versions bumped to 0.19.0.

Existing config.json files will keep their previous value (8) untouched ;
delete config.json to pick up the new default, or just bump it in
Settings → Avancés.

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

src/PSLauncher.App/ViewModels/SettingsViewModel.cs

@@ -49,6 +49,7 @@ public sealed partial class SettingsViewModel : ObservableObject
     [ObservableProperty] private string _language;
     [ObservableProperty] private string _reportUrl;
     [ObservableProperty] private string _documentationUrl;
+    [ObservableProperty] private int _parallelSegments;
 
     // ----- DB Migration settings -----
     [ObservableProperty] private string _dbHost;
@@ -173,6 +174,7 @@ public sealed partial class SettingsViewModel : ObservableObject
         _language = string.IsNullOrWhiteSpace(config.Language) ? "auto" : config.Language;
         _reportUrl = config.ReportUrl;
         _documentationUrl = config.DocumentationUrl;
+        _parallelSegments = config.ParallelDownloadSegments;
 
         _dbHost = config.Database.Host;
         _dbPort = config.Database.Port;
@@ -303,6 +305,7 @@ public sealed partial class SettingsViewModel : ObservableObject
         _config.Language = Language ?? "auto";
         _config.ReportUrl = ReportUrl?.Trim() ?? "";
         _config.DocumentationUrl = DocumentationUrl?.Trim() ?? "";
+        _config.ParallelDownloadSegments = Math.Clamp(ParallelSegments, 1, 32);
 
         // DB settings
         _config.Database.Host = DbHost.Trim();