fix: Handle UTF-8 BOM in config loading and build script

- main.js: Strip UTF-8 BOM when reading config.json
- build-client.bat: Write config.json without BOM using
  [System.Text.UTF8Encoding]::new($false)

PowerShell's -Encoding UTF8 writes BOM by default, which can
cause JSON parsing issues in Node.js.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

client/src/main.js

@@ -43,7 +43,12 @@ function loadConfig() {
     console.log(`Checking: ${configPath} - exists: ${exists}`);
     try {
       if (exists) {
-        const configData = fs.readFileSync(configPath, "utf-8");
+        // Use utf-8 and strip BOM if present (Windows Notepad adds BOM)
+        let configData = fs.readFileSync(configPath, "utf-8");
+        // Remove UTF-8 BOM if present
+        if (configData.charCodeAt(0) === 0xFEFF) {
+          configData = configData.slice(1);
+        }
         appConfig = JSON.parse(configData);
         console.log("Config loaded from:", configPath);
         console.log("Config content:", JSON.stringify(appConfig, null, 2));