feat: 토요일 연장근무 계산 기능 추가

- ShiftRepository에 getSaturdayOvertimeHours() 메소드 추가
- FragmentSettingsLab에 토요일 연장근무 시간 표시 카드 추가
- 토요일 수 * 2시간 계산 로직 구현

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)
Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

app/src/main/java/com/example/shiftalarm/FragmentSettingsLab.kt

@@ -31,6 +31,7 @@ class FragmentSettingsLab : Fragment() {
         
         setupSpinner()
         loadAnnualLeave()
+        loadSaturdayOvertime()
     }
 
     private fun setupSpinner() {
@@ -106,6 +107,14 @@ class FragmentSettingsLab : Fragment() {
         }
     }
 
+    private fun loadSaturdayOvertime() {
+        lifecycleScope.launch {
+            val repo = ShiftRepository(requireContext())
+            val overtimeHours = repo.getSaturdayOvertimeHours()
+            binding.tvSaturdayOvertime.text = "$overtimeHours"
+        }
+    }
+
     override fun onDestroyView() {
         super.onDestroyView()
         _binding = null