Fix export functionality and improve Windows build

- Fixed export function issues with comprehensive pandoc detection
- Added built-in HTML and PDF export fallbacks (no pandoc required)
- Improved error handling with clear user messages
- Enhanced code quality with optional chaining and better error handling
- Fixed NSIS installer script issues for Windows build
- Added comprehensive logging and debugging for export process
- Created test files and documentation for export functionality
- Successfully builds Windows release with proper XLSX support

Resolves export failures and provides robust fallback mechanisms.

test-export.md

@@ -0,0 +1,26 @@
+# Test Document
+
+This is a test markdown document for export testing.
+
+## Features
+
+- **Bold text**
+- *Italic text*
+- [Links](https://example.com)
+
+## Code Example
+
+```javascript
+function hello() {
+    console.log("Hello World!");
+}
+```
+
+## Table
+
+| Column 1 | Column 2 | Column 3 |
+|----------|----------|----------|
+| Value 1  | Value 2  | Value 3  |
+| Value 4  | Value 5  | Value 6  |
+
+This is a test document to verify export functionality.