fix(mobile): apply useModalStyle to remaining dialog files (Bug #9)

Completes the mobile fullscreen modal pass — adds useModalStyle to
DeviceVerificationSetup, DeviceVerificationReset, AddExistingModal,
RoomEncryption prompt, RoomUpgradeDialog, Modal500, ReadReceiptAvatars,
and RoomTopicViewer. All floating Dialog/Modal components now go
fullscreen on mobile (≤750px). UIAFlowOverlay was already fullscreen
via <Overlay>; JoinRulesSwitcher/RoomNotificationSwitcher are dropdowns.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/components/Modal500.tsx

@@ -2,12 +2,14 @@ import React, { ReactNode } from 'react';
 import FocusTrap from 'focus-trap-react';
 import { Modal, Overlay, OverlayBackdrop, OverlayCenter } from 'folds';
 import { stopPropagation } from '../utils/keyboard';
+import { useModalStyle } from '../hooks/useModalStyle';
 
 type Modal500Props = {
   requestClose: () => void;
   children: ReactNode;
 };
 export function Modal500({ requestClose, children }: Modal500Props) {
+  const modalStyle = useModalStyle(560);
   return (
     <Overlay open backdrop={<OverlayBackdrop />}>
       <OverlayCenter>
@@ -19,7 +21,7 @@ export function Modal500({ requestClose, children }: Modal500Props) {
             escapeDeactivates: stopPropagation,
           }}
         >
-          <Modal size="500" variant="Background">
+          <Modal size="500" variant="Background" style={modalStyle}>
             {children}
           </Modal>
         </FocusTrap>