Fix notification deduplication to prevent pile-up in navbar (#945)

* Fix notification deduplication to prevent pile-up in navbar (#944)

When tasks/projects remained pending for multiple days, duplicate
notifications accumulated in the navbar instead of showing only the
most recent one.

Updated notification services to properly handle existing notifications:
- Delete unread notifications before creating new ones
- Respect dismissed notifications (don't recreate)
- Respect read notifications (don't duplicate)

Changes:
- Updated dueTaskService, deferredTaskService, and dueProjectService
- Added comprehensive unit tests with 8 test cases

Fixes #944

* Fix lint errors and add GitHub bug template reminder to docs

backend/modules/tasks/deferredTaskService.js

@@ -68,7 +68,19 @@ async function checkDeferredTasks() {
                 );
 
                 if (existingNotification) {
-                    continue;
+                    // If notification was dismissed, don't create it again
+                    if (existingNotification.dismissed_at) {
+                        continue;
+                    }
+
+                    // If notification is unread, delete it before creating the new one
+                    // This prevents duplicate notifications from piling up
+                    if (!existingNotification.read_at) {
+                        await existingNotification.destroy();
+                    } else {
+                        // If it was already read, skip creating a new one
+                        continue;
+                    }
                 }
 
                 const sources = [];