feat(reminders): add ActionItem.reminder_set field (T8.6, FR-CAL-5)

src-tauri/src/models.rs

@@ -140,6 +140,9 @@ pub struct ActionItem {
     pub owner: Option<String>,
     pub due_at: Option<i64>,
     pub confirmed: bool,
+    /// Schedule a local OS reminder for `due_at` (Phase 8, T8.6, FR-CAL-5).
+    #[serde(default)]
+    pub reminder_set: bool,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]