Update CI workflow and add main entry point

Modified the CI workflow to correctly specify the path to the main script for PyInstaller builds. Added a new standalone entry point in src/main.py to serve as the executable script for tzst.

.github/workflows/ci.yml

@@ -173,7 +173,7 @@ jobs:
 
       - name: Build binary
         run: |
-          python -m PyInstaller --onefile --name tzst --console -m tzst
+          python -m PyInstaller --onefile --name tzst --console src/main.py
 
       - name: Verify binary (Linux/macOS)
         if: matrix.os != 'windows-latest'