Add option to manually swap shifts after the schedule has been generated

src-tauri/src/errors.rs

@@ -47,6 +47,29 @@ pub enum ExportError {
     OpenFailed(String),
 }
 
+#[derive(Error, Debug, Serialize)]
+#[serde(tag = "kind", content = "details")]
+pub enum SwapError {
+    #[error("no schedule has been generated yet")]
+    NotGenerated,
+    #[error("invalid shift position: {0}")]
+    InvalidPosition(String),
+    #[error("slot has no assigned resident")]
+    SlotEmpty,
+    #[error("both slots belong to the same resident")]
+    SameResident,
+    #[error("resident not found in config")]
+    ResidentNotFound,
+    #[error("resident cannot take the target slot: negative shift or disallowed type")]
+    ResidentCannotTakeSlot,
+    #[error("swap violates same-day or toxic pair constraint")]
+    SameDayOrToxicPair,
+    #[error("swap creates a consecutive-day constraint violation")]
+    ConsecutiveDays,
+    #[error("cannot swap the same slot or slots on the same day")]
+    SameSlotOrDay,
+}
+
 impl Serialize for ExportError {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
     where