Fix critical double-click file opening bug (v1.7.4)

CRITICAL BUG FIX: Files opened via double-click were not rendering in the application.

Root Cause:
- The openFileFromPath() function was sending IPC 'file-opened' message WITHOUT checking if rendererReady was true
- When files were double-clicked before renderer initialization, the IPC message was sent but lost
- The comment "Always try to send, the renderer will handle it when ready" was incorrect

The Fix (src/main.js:1586):
- Added rendererReady check to the conditional: if (mainWindow && mainWindow.webContents && rendererReady)
- Now properly stores file in app.pendingFile when renderer is not ready
- File is opened correctly when renderer-ready signal arrives

Flow:
1. File double-clicked → openFileFromPath() called
2. If renderer NOT ready → store in app.pendingFile
3. When renderer sends 'renderer-ready' → openFileFromPath(app.pendingFile) is called
4. This time renderer IS ready → file opens successfully

Version: 1.7.4
- Updated package.json version to 1.7.4
- Updated About dialog version to 1.7.4

🤖 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": "1.7.3",
+  "version": "1.7.4",
   "description": "Cross-platform Markdown editor and converter using Pandoc",
   "main": "src/main.js",
   "scripts": {