style(FN-1847): unify dialog overlay styling and add accessibility attributes

- Add role=dialog and aria-modal attributes to all modal dialogs for accessibility
- Standardize close button aria-labels across all modals
- Unify overlay backdrop-filter, z-index, and background styles in styles.css
- Add light theme overrides for non-standard overlay backgrounds
- Update AgentListModal tests to reflect new aria attributes
- Add changeset for @gsxdsm/fusion package

packages/dashboard/app/components/ActivityLogModal.tsx

@@ -180,6 +180,8 @@ export function ActivityLogModal({
       onClick={(e) => {
         if (e.target === e.currentTarget) onClose();
       }}
+      role="dialog"
+      aria-modal="true"
       data-testid="activity-log-modal-overlay"
     >
       <div className="modal modal-lg activity-log-modal" data-testid="activity-log-modal">