feat: Enhance validation viewing functionality with event emission

- Updated the `createMarkViewedHandler` to emit an event when a validation is marked as viewed, allowing the UI to update the unviewed count dynamically.
- Modified the `useUnviewedValidations` hook to handle the new event type for decrementing the unviewed validations count.
- Introduced a new event type `issue_validation_viewed` in the issue validation event type definition for better event handling.

apps/ui/src/components/layout/sidebar/hooks/use-unviewed-validations.ts

@@ -45,7 +45,11 @@ export function useUnviewedValidations(currentProject: Project | null) {
       const unsubscribe = api.github.onValidationEvent((event) => {
         if (event.projectPath === currentProject.path) {
           if (event.type === 'issue_validation_complete') {
+            // New validation completed - increment count
             setCount((prev) => prev + 1);
+          } else if (event.type === 'issue_validation_viewed') {
+            // Validation was viewed - decrement count
+            setCount((prev) => Math.max(0, prev - 1));
           }
         }
       });