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/components/HomeButton.vue

@@ -6,7 +6,10 @@ const goHome = () => router.push({ name: "Home" })
 </script>
 
 <template>
-  <button class="btn btn-ghost btn-circle btn-lg text-base-content" @click="goHome">
+  <button
+    class="btn btn-ghost btn-circle btn-lg text-base-content"
+    @click="goHome"
+  >
     <img src="/favicon.png" alt="Remanso icon" class="remanso-logo" />
   </button>
 </template>