refactor: migrate Windows target from x86 to x64 architecture

Update all Windows build configurations, scripts, and documentation from i686-pc-windows-msvc (x86) to x86_64-pc-windows-msvc (x64). Update npm scripts, build-tunnel-helper.sh target validation, bundled helper paths, and tunnel manager strategy references. Add XWIN_ARCH=x86_64 environment variable to Linux cross-build command and --xwin-arch flag to cargo xwin invocation.

README.md

@@ -46,7 +46,7 @@ This repository contains the initial production-minded MVP scaffold:
 
 ## Desktop Requirements
 
-- Windows x86: package NexaVPN with the bundled Windows x86 tunnel helper
+- Windows x64: package NexaVPN with the bundled Windows x64 tunnel helper
 - macOS ARM: package NexaVPN with the bundled macOS ARM tunnel helper
 
 See [client-platforms.md](/mnt/c/Users/neste/Documents/GIT/NexaVPN/docs/client-platforms.md) for the current platform strategy.
@@ -55,7 +55,7 @@ Helper build commands:
 
 ```bash
 cd desktop-client
-npm run helper:windows-x86
+npm run helper:windows-x64
 npm run helper:macos-arm64
 ```
 
@@ -64,17 +64,19 @@ Ubuntu-to-Windows `Setup.exe` build:
 ```bash
 cd desktop-client
 cargo install --locked cargo-xwin
-rustup target add i686-pc-windows-msvc
+rustup target add x86_64-pc-windows-msvc
 npm install
-npm run helper:windows-x86
-npm run tauri:build:windows-x86:linux
+npm run helper:windows-x64
+npm run tauri:build:windows-x64:linux
 ```
 
 The resulting NSIS installer is written to:
 
 - `desktop-client/src-tauri/target/i686-pc-windows-msvc/release/bundle/nsis/`
+ - `desktop-client/src-tauri/target/x86_64-pc-windows-msvc/release/bundle/nsis/`
 
 This Linux cross-build path is intended for NSIS `Setup.exe` output. Native MSI packaging and final Windows code signing should still be done on Windows.
+For `x86_64-pc-windows-msvc`, the build uses the Windows `x86_64` SDK/CRT set via `cargo-xwin`.
 
 Gateway utility scripts: