feat: bump to v4.1.0 and add CI/CD release pipeline

- Bump version to 4.1.0 in package.json and index.html
- Add build:local script for combined Linux + Windows local builds
- Add CI workflow: runs tests on push/PR to master
- Add Release workflow: tag-triggered (v*), parallel Linux + Windows
  builds, publishes all packages to GitHub Releases

Amit Haridas

package.json

@@ -1,6 +1,6 @@
 {
   "name": "markdown-converter",
-  "version": "4.0.0",
+  "version": "4.1.0",
   "description": "Professional Markdown editor and universal file converter with PDF editing, batch processing, and syntax highlighting",
   "main": "src/main.js",
   "scripts": {
@@ -18,6 +18,7 @@
     "build:win-unsigned": "cross-env CSC_IDENTITY_AUTO_DISCOVERY=false electron-builder --win",
     "build:mac": "electron-builder --mac",
     "build:linux": "electron-builder --linux",
+    "build:local": "electron-builder --linux --win",
     "dist": "electron-builder --publish=never",
     "dist:all": "electron-builder -mwl",
     "generate-icons": "node scripts/generate-icons.js"