fix: exponential backoff for 429 retries, cover thumbnails, 2 more modal dialogs

- matrix.ts: rateLimitedActions fallback delay now uses capped exponential
  backoff (min(1000 * 2^n, 30s)) instead of flat 3000ms when server omits
  Retry-After; server header still takes precedence
- RenderMessageContent: add objectFit:cover + 100% fill to video thumbnail
  <Image> so thumbnails fill their container without letterboxing (P5-6)
- CreateRoomModal, CreateSpaceModal: apply useModalStyle(480) for fullscreen
  on mobile
- LOTUS_BUGS: mark usePan memory leak + httpStatus check as FALSE POSITIVE;
  mark rateLimitedActions backoff as FIXED

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/features/create-space/CreateSpaceModal.tsx

@@ -23,6 +23,7 @@ import {
 } from '../../state/hooks/createSpaceModal';
 import { CreateSpaceModalState } from '../../state/createSpaceModal';
 import { stopPropagation } from '../../utils/keyboard';
+import { useModalStyle } from '../../hooks/useModalStyle';
 
 type CreateSpaceModalProps = {
   state: CreateSpaceModalState;
@@ -30,6 +31,7 @@ type CreateSpaceModalProps = {
 function CreateSpaceModal({ state }: CreateSpaceModalProps) {
   const { spaceId } = state;
   const closeDialog = useCloseCreateSpaceModal();
+  const modalStyle = useModalStyle(480);
 
   const allJoinedRooms = useAllJoinedRoomsSet();
   const getRoom = useGetRoom(allJoinedRooms);
@@ -47,7 +49,7 @@ function CreateSpaceModal({ state }: CreateSpaceModalProps) {
               escapeDeactivates: stopPropagation,
             }}
           >
-            <Modal size="300" flexHeight>
+            <Modal size="300" flexHeight style={modalStyle}>
               <Box direction="Column">
                 <Header
                   size="500"