feat: Add validation viewing functionality and UI updates

- Implemented a new function to mark validations as viewed by the user, updating the validation state accordingly.
- Added a new API endpoint for marking validations as viewed, integrated with the existing GitHub routes.
- Enhanced the sidebar to display the count of unviewed validations, providing real-time updates.
- Updated the GitHub issues view to mark validations as viewed when issues are accessed, improving user interaction.
- Introduced a visual indicator for unviewed validations in the issue list, enhancing user awareness of pending validations.

libs/types/src/issue-validation.ts

@@ -123,4 +123,6 @@ export interface StoredValidation {
   model: AgentModel;
   /** The validation result */
   result: IssueValidationResult;
+  /** ISO timestamp when user viewed this validation (undefined = not yet viewed) */
+  viewedAt?: string;
 }