feat: Add QR code generation functionality to PrivateBin API library

- Add generate_qr_code() function for creating QR codes from PrivateBin links
- Implement SVG output format for scalable QR code generation
- Add qr_generator.h/cpp with self-contained QR code implementation
- Update CMakeLists.txt to include new QR code source files
- Integrate QR code documentation into README.md
- Update example program to demonstrate QR code generation
- Update CHANGELOG.md with v0.1.1.6 release notes
- Remove separate README_QR_CODE.md file

The implementation provides simplified QR code generation with:
- Position detection patterns for QR code recognition
- Configurable size and border parameters
- No external dependencies
- Comprehensive error handling
- Memory management using existing free_string() function

include/qr_generator.h

@@ -0,0 +1,28 @@
+#ifndef QR_GENERATOR_H
+#define QR_GENERATOR_H
+
+#include <vector>
+#include <string>
+
+namespace QRGenerator {
+    /**
+     * Generates QR code data for a given URL in SVG format
+     * 
+     * @param url The URL to encode
+     * @param size The size of the QR code in pixels
+     * @param border The border size around the QR code in modules
+     * @return String containing the QR code data in SVG format
+     */
+    std::string generate_qr_code_svg(const std::string& url, int size = 256, int border = 4);
+    
+    /**
+     * Generates a simple text-based QR code representation
+     * 
+     * @param url The URL to encode
+     * @param width The width of the text representation
+     * @return String containing the text-based QR code
+     */
+    std::string generate_qr_code_text(const std::string& url, int width = 40);
+}
+
+#endif // QR_GENERATOR_H