Add client requirements file with pynput for global input capture

clients/client_requirements.txt

@@ -0,0 +1,14 @@
+# NFC Network Client Requirements
+# Install with: pip install -r client_requirements.txt
+
+# GUI Framework
+tkinter-tooltip>=2.2.0; sys_platform != "linux"
+
+# HTTP Client
+requests>=2.31.0
+
+# Global Input Capture - REQUIRED for background NFC scanning
+pynput>=1.7.6
+
+# Type hints support
+typing-extensions>=4.5.0