fix(ui): ScheduleMessageModal + PollCreator use folds Dialog shell (N15, N29)

Both rendered as <Box as="form" role="dialog"> with manually assembled
background/borderRadius(R400)/boxShadow. Switch to <Dialog as="form"
variant="Surface"> so the surface comes from the design system (R300 radius),
matching the other message-action dialogs.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/features/room/PollCreator.tsx

@@ -3,6 +3,7 @@ import FocusTrap from 'focus-trap-react';
 import {
   Box,
   Button,
+  Dialog,
   Header,
   Icon,
   IconButton,
@@ -100,19 +101,14 @@ export function PollCreator({ roomId, onClose }: PollCreatorProps) {
             escapeDeactivates: stopPropagation,
           }}
         >
-          <Box
+          <Dialog
             as="form"
+            variant="Surface"
             role="dialog"
             aria-modal="true"
             aria-labelledby="poll-creator-title"
             onSubmit={handleSubmit}
-            direction="Column"
-            style={{
-              background: color.Surface.Container,
-              borderRadius: config.radii.R400,
-              boxShadow: color.Other.Shadow,
-              ...modalStyle,
-            }}
+            style={modalStyle}
           >
             {/* Header */}
             <Header
@@ -256,7 +252,7 @@ export function PollCreator({ roomId, onClose }: PollCreatorProps) {
                 </Button>
               </Box>
             </Box>
-          </Box>
+          </Dialog>
         </FocusTrap>
       </OverlayCenter>
     </Overlay>

src/app/features/room/ScheduleMessageModal.tsx

@@ -3,6 +3,7 @@ import FocusTrap from 'focus-trap-react';
 import {
   Box,
   Button,
+  Dialog,
   Header,
   Icon,
   IconButton,
@@ -177,19 +178,14 @@ export function ScheduleMessageModal({
             escapeDeactivates: stopPropagation,
           }}
         >
-          <Box
+          <Dialog
             as="form"
+            variant="Surface"
             role="dialog"
             aria-modal="true"
             aria-labelledby="schedule-message-title"
             onSubmit={handleSubmit}
-            direction="Column"
-            style={{
-              background: color.Surface.Container,
-              borderRadius: config.radii.R400,
-              boxShadow: color.Other.Shadow,
-              ...modalStyle,
-            }}
+            style={modalStyle}
           >
             {/* Header */}
             <Header
@@ -341,7 +337,7 @@ export function ScheduleMessageModal({
                 <Text size="B400">Schedule</Text>
               </Button>
             </Box>
-          </Box>
+          </Dialog>
         </FocusTrap>
       </OverlayCenter>
     </Overlay>