refactor: reduce window size and remove transfer metrics display from main UI

Reduce default window dimensions from 1120x760 to 940x640 pixels and disable resizing. Remove TunnelMetrics and RawTunnelMetrics types, formatDataSize and normalizeTunnelMetrics helpers, and all transfer statistics tracking from App component. Replace refreshTunnelMetrics with simpler refreshTunnelStatus that only queries tunnel active state. Remove received/sent data display cards from status panel and eliminate metrics

desktop-client/src-tauri/tauri.conf.json

@@ -13,8 +13,8 @@
     "windows": [
       {
         "title": "NexaVPN",
-        "width": 1120,
-        "height": 760,
+        "width": 940,
+        "height": 640,
         "resizable": false,
         "maximizable": false
       }