installer: switch Windows target from NSIS to portable single-exe

Both electron-builder configs now produce a single-file portable .exe
instead of an NSIS installer. Removes installer/uninstaller icons and
the install-directory wizard — users can run the .exe directly.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

electron-builder.yml

@@ -23,12 +23,8 @@ extraResources:
     filter:
       - "**/*"
 win:
-  target: nsis
-  artifactName: ${productName}-${version}-Setup.${ext}
+  target: portable
+  artifactName: ${productName}-${version}-Portable.${ext}
   icon: build/icon.ico
-nsis:
-  oneClick: false
-  allowToChangeInstallationDirectory: true
-  perMachine: false
-  installerIcon: build/icon.ico
-  uninstallerIcon: build/icon.ico
+portable:
+  artifactName: ${productName}-${version}-Portable.${ext}