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

CMakeLists.txt

@@ -35,6 +35,7 @@ set(SOURCES
     src/crypto.cpp
     src/json_parser.cpp
     src/base58.cpp
+    src/qr_generator.cpp
 )
 
 set(HEADERS
@@ -43,6 +44,7 @@ set(HEADERS
     include/crypto.h
     include/json_parser.h
     include/base58.h
+    include/qr_generator.h
 )
 
 # Create the shared library