Fix print preview functionality and set GitHub as default theme

- Fixed print-preview event handling by adding missing ipcRenderer listeners
- Renderer now properly relays print-preview and print-preview-styled events to main process
- Changed default theme from 'light' to 'github' for better visual appearance
- Rebuilt release with v1.7.7 fixes

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

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

src/main.js

@@ -1157,7 +1157,7 @@ ipcMain.on('save-current-file', (event, content) => {
 });
 
 ipcMain.on('get-theme', (event) => {
-  const theme = store.get('theme', 'light');
+  const theme = store.get('theme', 'github');
   event.reply('theme-changed', theme);
 });