Fix v1.3.1: File associations and 50/50 pane layout

- Enhanced file association handling for double-clicking .md files
- Added proper file associations in package.json build config
- Fixed preview/source pane layout to be equally distributed 50/50
- Updated CSS for tab-content structure with proper flex layout
- Added theme support for new pane selectors
- Improved command line argument processing for file opening
- Updated version to 1.3.1 with file association feature

package.json

@@ -1,6 +1,6 @@
 {
   "name": "pan-converter",
-  "version": "1.3.0",
+  "version": "1.3.1",
   "description": "Cross-platform Markdown editor and converter using Pandoc",
   "main": "src/main.js",
   "scripts": {
@@ -47,6 +47,22 @@
       "node_modules/**/*",
       "package.json"
     ],
+    "fileAssociations": [
+      {
+        "ext": "md",
+        "name": "Markdown Document",
+        "description": "Markdown Document",
+        "mimeType": "text/markdown",
+        "role": "Editor"
+      },
+      {
+        "ext": "markdown",
+        "name": "Markdown Document",
+        "description": "Markdown Document", 
+        "mimeType": "text/markdown",
+        "role": "Editor"
+      }
+    ],
     "mac": {
       "category": "public.app-category.productivity",
       "icon": "assets/icon.icns"