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/MilestoneSliceInterviewModal.tsx

@@ -448,6 +448,8 @@ export function MilestoneSliceInterviewModal({
     <div
       className="modal-overlay open"
       onClick={(e) => e.target === e.currentTarget && handleCancel()}
+      role="dialog"
+      aria-modal="true"
       data-testid="milestone-slice-interview-modal"
     >
       <div className="modal modal-lg planning-modal">