Clarify active Windows client architecture

apps/windows-client/src-tauri/capabilities/default.json

@@ -0,0 +1,7 @@
+{
+  "$schema": "../gen/schemas/desktop-schema.json",
+  "identifier": "default",
+  "description": "Default capability for the main VPN Proxy Windows shell. Task 8 keeps helper/install launch explicit: no shell or sidecar permission is granted here until a packaged helper is declared.",
+  "windows": ["main"],
+  "permissions": ["core:default"]
+}