Fix critical bugs: real-time preview, toolbar functions, and file opening

Version 1.5.7 - Bug Fix Release

Fixed Issues:
- Real-time preview updates: Preview pane now updates instantly while typing
- Toolbar markdown functions: All toolbar buttons now work correctly
  * Bold, Italic, Code, Link text wrapping
  * Heading, List, Quote line prefixes
  * Table insertion
- File opening: Fixed file loading via menu and double-click association
- Cross-platform path handling for Windows backslashes

Technical Changes:
- Added direct input event listeners to editor textareas
- Implemented wrapSelection() and insertAtLineStart() helper methods
- Fixed IPC communication between main and renderer processes
- Improved event delegation for multi-tab support

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

test.md

@@ -0,0 +1,23 @@
+# Test Document
+
+This is a **test** markdown document.
+
+## Features
+
+- Item 1
+- Item 2
+- Item 3
+
+### Code Example
+
+```javascript
+console.log("Hello World");
+```
+
+> Tqweqweqweqweqweqweqeweqweqweqwe
+dfe
+
+qwe
+qwe
+qwe
+qwe