v0.21.0 — System health banner with colored indicators + tooltips

New row between top bar and content showing pill-shaped status indicators
for the system dependencies that PROSERVE needs (SteamVR, Vive Business
Streaming process, VR headset reachable on the network, etc.). Each pill
is colored : 🟢 OK, 🟠 limitation détectée (e.g. ping élevé), 🔴 KO,
gris = non configuré. Hover → tooltip with full check kind, target,
last RTT/process count and timestamp.

Architecture
- LocalConfig.HealthChecksConfig : list of HealthCheckEntry (Kind=Ping
  or Process, Target=IP/host or process name, ping thresholds in ms,
  refresh interval in s).
- ISystemHealthService + SystemHealthService : ping via
  System.Net.NetworkInformation.Ping (no admin required), process
  lookup via System.Diagnostics.Process.GetProcessesByName. Returns
  HealthResult { Severity, Detail }.
- HealthIndicatorViewModel : observable wrapper per entry with
  Severity-driven Brushes (background pill, border, icon colour) and
  composed Tooltip text.
- MainViewModel.InitHealthIndicators + StartHealthLoop : populates
  ObservableCollection<HealthIndicatorViewModel> from config and runs
  parallel checks every RefreshIntervalSeconds (default 10s).
- MainWindow.xaml : new Row 1 (between top bar and body) hosting an
  ItemsControl bound to HealthIndicators. Row collapses cleanly if the
  list is empty (HasHealthIndicators=false).

Defaults shipped (all editable in %LocalAppData%\PSLauncher\config.json)
- 🎮 SteamVR (Process : vrserver)
- 📡 Vive Business Streaming (Process : HtcConnectionUtility)
- 🥽 Casque VR (Ping : empty, user fills in the headset IP)

The Settings UI editor for managing the list is intentionally deferred
to a future iteration — config.json edit is enough to start using it.

Versions bumped to 0.21.0.

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     "0.20.0"
+#define MyAppVersion     "0.21.0"
 #define MyAppPublisher   "ASTERION VR"
 #define MyAppURL         "https://asterionvr.com"
 #define MyAppExeName     "PS_Launcher.exe"