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/App.vue

@@ -25,6 +25,12 @@ const { isATProtoReady } = useATProtoLogin()
   overflow-x: auto;
 }
 
+@media screen and (max-width: 768px) {
+  #main-app {
+    overflow-y: auto;
+  }
+}
+
 ::view-transition-old(root),
 ::view-transition-new(root) {
   animation-duration: 0.25s;