refactor: remove direct Windows metrics from tunnel_manager and update wg.exe fallback in tunnel-helper

Remove direct_windows_metrics, read_windows_metrics_from_show, parse_human_wireguard_bytes, and find_windows_wg functions from tunnel_manager.rs to rely on bundled backend for all metrics queries. Update find_wg_cli in tunnel-helper to return "wg.exe" as fallback when WireGuard installation paths don't exist, removing "wg" from candidate list.

desktop-client/tunnel-helper/src/main.rs

@@ -735,15 +735,15 @@ fn read_transfer_totals(profile: &Path) -> Result<(u64, u64), String> {
 #[cfg(target_os = "windows")]
 fn find_wg_cli() -> Result<PathBuf, String> {
     let candidates = [
-        PathBuf::from("wg"),
         PathBuf::from(r"C:\Program Files\WireGuard\wg.exe"),
         PathBuf::from(r"C:\Program Files (x86)\WireGuard\wg.exe"),
     ];
 
-    candidates
-        .into_iter()
-        .find(|path| path.exists())
-        .ok_or_else(|| "required Windows WireGuard CLI is not available".to_string())
+    if let Some(path) = candidates.into_iter().find(|path| path.exists()) {
+        return Ok(path);
+    }
+
+    Ok(PathBuf::from("wg.exe"))
 }
 
 #[cfg(target_os = "macos")]