Fix compiler warnings and improve code quality

- Replace unsafe strcpy with strcpy_s for better security
- Fix DLL binding issues by adding PRIVATEBIN_API macros
- Add explicit type casts to resolve size conversion warnings
- Replace unsafe getenv with _dupenv_s for better security
- Add PRIVATEBINAPI_EXPORTS definition in CMakeLists.txt
- Improve CMake configuration for better build compatibility

tests/test_basic.cpp

@@ -21,9 +21,12 @@ static bool extract_paste_id_and_key(const std::string& full_url, std::string& p
 }
 
 int main() {
-    const char* it = std::getenv("PRIVATEBIN_IT");
+    char* it = nullptr;
+    size_t len = 0;
+    _dupenv_s(&it, &len, "PRIVATEBIN_IT");
     if (!it || std::string(it) == "0") {
         std::cout << "[test] PRIVATEBIN_IT not set; skipping integration test." << std::endl;
+        free(it);
         return 0; // treat as success when integration testing is disabled
     }