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/ChangesDiffModal.tsx

@@ -112,7 +112,7 @@ export function ChangesDiffModal({
   const isDone = column === "done";
 
   return (
-    <div className="modal-overlay open" onClick={onClose}>
+    <div className="modal-overlay open" onClick={onClose} role="dialog" aria-modal="true">
       <div
         className="modal changes-diff-modal"
         onClick={(e) => e.stopPropagation()}
@@ -171,7 +171,7 @@ export function ChangesDiffModal({
                 Refresh
               </button>
             )}
-            <button className="modal-close" onClick={onClose}>
+            <button className="modal-close" onClick={onClose} aria-label="Close">
               <X size={20} />
             </button>
           </div>