fix: use electron-builder Wine image, disable code signing

- Runner now uses electronuserland/builder:20-wine which has Node 20,
  Wine, and all build tools pre-installed
- Removed manual Wine/Node setup steps from workflow
- Disabled Windows code signing (no certificate available)
- Simplified workflow to: checkout -> npm ci -> build -> package -> release

electron-builder.yml

@@ -29,6 +29,8 @@ asar: true
 win:
   target: nsis
   icon: assets/icon.ico
+  sign: false
+  signingHashAlgorithms: null
 nsis:
   oneClick: false
   allowToChangeInstallationDirectory: true