Fix print preview to show content and improve file loading timing

Print Preview Fixes:
- Fixed print preview to show actual preview content instead of formatting menu
- Changed from generic 'preview' ID to active tab's 'preview-{tabId}' element
- Added CSS positioning to make preview fixed and full-screen during print
- Hide all dialog overlays (export, batch, PDF, converter) during print
- Restore dialogs after print completes
- Enhanced print-mode CSS with z-index and positioning for proper display

File Loading Timing Improvements:
- Increased renderer-ready delay from 500ms to 1000ms for slower first loads
- Increased openFileFromPath delay from 100ms to 300ms
- Ensures UI fully initializes before files are displayed
- Files now render correctly on first double-click launch

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

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

src/main.js

@@ -1668,11 +1668,11 @@ function openFileFromPath(filePath) {
     currentFile = filePath;
     const content = fs.readFileSync(filePath, 'utf-8');
     if (mainWindow && mainWindow.webContents && rendererReady) {
-      // Add a small delay to ensure renderer is fully prepared to display the file
-      // This prevents timing issues where file opens before UI is ready
+      // Add delay to ensure renderer is fully prepared to display the file
+      // Increased delay for first-time load when UI is still initializing
       setTimeout(() => {
         mainWindow.webContents.send('file-opened', { path: filePath, content });
-      }, 100);
+      }, 300);
     } else {
       // Store file to open after renderer is ready
       app.pendingFile = filePath;