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/room-topic-viewer/RoomTopicViewer.tsx

@@ -1,6 +1,7 @@
 import React from 'react';
 import parse from 'html-react-parser';
 import { as, Box, Header, Icon, IconButton, Icons, Modal, Scroll, Text } from 'folds';
+import { useModalStyle } from '../../hooks/useModalStyle';
 import classNames from 'classnames';
 import Linkify from 'linkify-react';
 import * as css from './style.css';
@@ -17,6 +18,7 @@ export const RoomTopicViewer = as<
   }
 >(({ name, topic, requestClose, className, ...props }, ref) => {
   const topicStr = typeof topic === 'string' ? topic : topic.topic;
+  const modalStyle = useModalStyle(480);
   const isFormatted =
     typeof topic !== 'string' &&
     topic.format === 'org.matrix.custom.html' &&
@@ -28,6 +30,7 @@ export const RoomTopicViewer = as<
       flexHeight
       className={classNames(css.ModalFlex, className)}
       aria-labelledby="room-topic-title"
+      style={modalStyle}
       {...props}
       ref={ref}
     >