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/features/add-existing/AddExisting.tsx

@@ -54,6 +54,7 @@ import { StateEvent } from '../../../types/matrix/room';
 import { getViaServers } from '../../plugins/via-servers';
 import { rateLimitedActions } from '../../utils/matrix';
 import { useAlive } from '../../hooks/useAlive';
+import { useModalStyle } from '../../hooks/useModalStyle';
 
 const SEARCH_OPTS: UseAsyncSearchOptions = {
   limit: 500,
@@ -72,6 +73,7 @@ type AddExistingModalProps = {
 };
 export function AddExistingModal({ parentId, space, requestClose }: AddExistingModalProps) {
   const mx = useMatrixClient();
+  const modalStyle = useModalStyle(480);
   const useAuthentication = useMediaAuthentication();
   const alive = useAlive();
 
@@ -188,7 +190,7 @@ export function AddExistingModal({ parentId, space, requestClose }: AddExistingM
             escapeDeactivates: stopPropagation,
           }}
         >
-          <Modal size="300">
+          <Modal size="300" style={modalStyle}>
             <Box grow="Yes" direction="Column">
               <Header
                 size="500"