Fix double-click file opening in installed Windows app (v1.7.5)

This fix addresses the critical issue where double-clicking .md files would not open them in the installed PanConverter application on Windows.

## Root Cause
When a user double-clicks a file and the app is already running, Windows tries to launch a SECOND INSTANCE with the file path. Without single-instance lock handling, that second instance would exit and the first instance would never receive the file path.

## Changes
- Added single-instance lock handling in main.js (lines 52-85)
- Implemented second-instance event handler to capture file paths
- Focus existing window when second instance is attempted
- Pass file path to existing instance using openFileFromPath()
- Properly handle rendererReady state for file opening

## Testing
- Works in development mode (npm start)
- Should now work correctly in installed Windows app with double-click
- Maintains single instance behavior across all file opening methods

🤖 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": "1.7.4",
+  "version": "1.7.5",
   "description": "Cross-platform Markdown editor and converter using Pandoc",
   "main": "src/main.js",
   "scripts": {