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/common-settings/general/RoomEncryption.tsx

@@ -28,6 +28,7 @@ import { useRoom } from '../../../hooks/useRoom';
 import { useStateEvent } from '../../../hooks/useStateEvent';
 import { stopPropagation } from '../../../utils/keyboard';
 import { RoomPermissionsAPI } from '../../../hooks/useRoomPermissions';
+import { useModalStyle } from '../../../hooks/useModalStyle';
 
 const ROOM_ENC_ALGO = 'm.megolm.v1.aes-sha2';
 
@@ -37,6 +38,7 @@ type RoomEncryptionProps = {
 export function RoomEncryption({ permissions }: RoomEncryptionProps) {
   const mx = useMatrixClient();
   const room = useRoom();
+  const modalStyle = useModalStyle(480);
 
   const canEnable = permissions.stateEvent(StateEvent.RoomEncryption, mx.getSafeUserId());
   const content = useStateEvent(room, StateEvent.RoomEncryption)?.getContent<{
@@ -111,7 +113,7 @@ export function RoomEncryption({ permissions }: RoomEncryptionProps) {
                   escapeDeactivates: stopPropagation,
                 }}
               >
-                <Dialog variant="Surface">
+                <Dialog variant="Surface" style={modalStyle}>
                   <Header
                     style={{
                       padding: `0 ${config.space.S200} 0 ${config.space.S400}`,