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/components/RenderMessageContent.tsx

@@ -232,7 +232,13 @@ export function RenderMessageContent({
                       <ThumbnailContent
                         info={info}
                         renderImage={(src) => (
-                          <Image alt={body} title={body} src={src} loading="lazy" />
+                          <Image
+                            alt={body}
+                            title={body}
+                            src={src}
+                            loading="lazy"
+                            style={{ objectFit: 'cover', width: '100%', height: '100%' }}
+                          />
                         )}
                       />
                     )