fix(mobile): restore overflow-y and unstick readme on vertical scroll

- Restore explicit overflow-y:auto on #main-app for mobile (removed in
  63f5d64) — implicit coercion from overflow-x:auto is not reliable in
  all Safari/WebKit versions.
- Override position:sticky on .readme to position:relative on mobile.
  The desktop sticky (left:0) is correct for horizontal scroll, but on
  mobile vertical scroll it pinned the 100dvh-tall readme across the
  entire viewport, hiding all stacked notes behind it.

src/views/RepoList.vue

@@ -8,8 +8,14 @@ import { useRepoList } from "@/modules/repo/hooks/useRepoList.hook"
 
 const { username } = useGitHubLogin()
 const { isReady } = useRepos()
-const { favoriteRepos, otherRepos, favoriteCheckboxes, toggleCheckbox, canLoadMore, loadMore } =
-  useRepoList()
+const {
+  favoriteRepos,
+  otherRepos,
+  favoriteCheckboxes,
+  toggleCheckbox,
+  canLoadMore,
+  loadMore
+} = useRepoList()
 </script>
 
 <template>