Add negative shifts in flex map logic and make it dynamic

src-tauri/src/slot.rs

@@ -136,6 +136,14 @@ impl Day {
         !self.0.is_multiple_of(2)
     }
 
+    pub fn shift_types(&self) -> &[ShiftType] {
+        if self.is_open_shift() {
+            &[ShiftType::OpenFirst, ShiftType::OpenSecond]
+        } else {
+            &[ShiftType::Closed]
+        }
+    }
+
     pub fn next(&self) -> Self {
         Self(self.0 + 1)
     }