Fix critical bugs in v1.7.8: file association and print preview

- Fixed command-line argument parsing for packaged apps using app.isPackaged
- Fixed print preview to show markdown content instead of toolbar
- Rewrote print handler to rely on CSS @media print rules
- Enhanced CSS with attribute selectors for dynamic preview panes
- Removed DevTools auto-open for production build
- Updated version to 1.7.8 across all files

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

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "pan-converter",
-  "version": "1.7.7",
+  "version": "1.7.8",
   "description": "Cross-platform Markdown editor and converter using Pandoc",
   "main": "src/main.js",
   "scripts": {