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>

src/main.js

@@ -49,6 +49,41 @@ let currentFile = null; // This will now represent the active tab's file
 let pandocAvailable = null; // Cache pandoc availability check
 let rendererReady = false; // Track if renderer is ready to receive file data
 
+// Handle single instance lock for Windows file association
+// When a file is double-clicked and the app is already running,
+// Windows tries to start a second instance. We prevent this and
+// pass the file to the existing instance instead.
+const gotTheLock = app.requestSingleInstanceLock();
+
+if (!gotTheLock) {
+  // Another instance is already running, quit this one
+  app.quit();
+} else {
+  // This is the first instance, handle second-instance events
+  app.on('second-instance', (event, commandLine, workingDirectory) => {
+    // Someone tried to run a second instance, focus our window instead
+    if (mainWindow) {
+      if (mainWindow.isMinimized()) mainWindow.restore();
+      mainWindow.focus();
+    }
+
+    // Check if a file was passed to the second instance
+    // commandLine is an array like: ['electron.exe', 'app.asar', 'file.md']
+    const fileArgs = commandLine.slice(2);
+    for (const arg of fileArgs) {
+      if ((arg.endsWith('.md') || arg.endsWith('.markdown')) && fs.existsSync(arg)) {
+        // Open the file in the existing instance
+        if (rendererReady) {
+          openFileFromPath(arg);
+        } else {
+          app.pendingFile = arg;
+        }
+        break;
+      }
+    }
+  });
+}
+
 // Check if pandoc is available
 function checkPandocAvailability() {
   return new Promise((resolve) => {