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/SubtaskBreakdownModal.tsx

@@ -515,7 +515,7 @@ export function SubtaskBreakdownModal({ isOpen, onClose, initialDescription, onT
   if (!isOpen) return null;
 
   return (
-    <div className="modal-overlay open" onClick={(event) => event.target === event.currentTarget && void handleClose()}>
+    <div className="modal-overlay open" onClick={(event) => event.target === event.currentTarget && void handleClose()} role="dialog" aria-modal="true">
       <div className="modal modal-lg planning-modal">
         <div className="modal-header">
           <div className="detail-title-row">