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

src/styles.css

@@ -152,8 +152,36 @@ body {
 .pane {
     flex: 1;
     overflow: auto;
+    min-width: 0;
 }
 
+.pane:first-child {
+    border-right: 1px solid #ddd;
+}
+
+.editor-textarea {
+    width: 100%;
+    height: 100%;
+    padding: 20px;
+    font-family: 'SF Mono', Monaco, 'Courier New', monospace;
+    font-size: 14px;
+    line-height: 1.6;
+    border: none;
+    outline: none;
+    resize: none;
+}
+
+.pane:last-child {
+    padding: 20px;
+    background: #fff;
+}
+
+.preview-content {
+    max-width: 800px;
+    margin: 0 auto;
+}
+
+/* Legacy support for old selectors */
 #editor-pane {
     border-right: 1px solid #ddd;
 }
@@ -356,6 +384,20 @@ body.theme-dark .toolbar-separator {
     background: #3e3e42;
 }
 
+body.theme-dark .pane:first-child {
+    border-right-color: #3e3e42;
+}
+
+body.theme-dark .editor-textarea {
+    background: #1e1e1e;
+    color: #d4d4d4;
+}
+
+body.theme-dark .pane:last-child {
+    background: #252526;
+}
+
+/* Legacy support */
 body.theme-dark #editor-pane {
     border-right-color: #3e3e42;
 }
@@ -574,6 +616,15 @@ body.theme-github .status-bar {
 }
 
 /* Hide preview pane by default */
+.pane.hidden {
+    display: none;
+}
+
+.pane.full-width {
+    border-right: none;
+}
+
+/* Legacy support */
 #preview-pane.hidden {
     display: none;
 }