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/read-receipt-avatars/ReadReceiptAvatars.tsx

@@ -12,6 +12,7 @@ import { UserAvatar } from '../user-avatar';
 import { StackedAvatar } from '../stacked-avatar';
 import { EventReaders } from '../event-readers';
 import { stopPropagation } from '../../utils/keyboard';
+import { useModalStyle } from '../../hooks/useModalStyle';
 
 const MAX_DISPLAY = 5;
 
@@ -28,6 +29,7 @@ export function ReadReceiptAvatars({
   const useAuthentication = useMediaAuthentication();
   const [open, setOpen] = useState(false);
   const [lotusTerminal] = useSetting(settingsAtom, 'lotusTerminal');
+  const modalStyle = useModalStyle(360);
 
   if (userIds.length === 0) return null;
 
@@ -51,7 +53,7 @@ export function ReadReceiptAvatars({
               escapeDeactivates: stopPropagation,
             }}
           >
-            <Modal variant="Surface" size="300">
+            <Modal variant="Surface" size="300" style={modalStyle}>
               <EventReaders room={room} eventId={eventId} requestClose={() => setOpen(false)} />
             </Modal>
           </FocusTrap>