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/utils/matrix.ts

@@ -364,7 +364,8 @@ export const rateLimitedActions = async <T, R = void>(
         return;
       }
 
-      const waitMS = err.getRetryAfterMs() ?? 3000;
+      // Respect server Retry-After header; fall back to capped exponential backoff.
+      const waitMS = err.getRetryAfterMs() ?? Math.min(1000 * 2 ** retryCount, 30_000);
       actionInterval = waitMS * 1.5;
       await sleepForMs(waitMS);
       retryCount += 1;