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/shims-vue.d.ts

@@ -1,5 +1,5 @@
-declare module '*.vue' {
-  import type { DefineComponent } from 'vue'
+declare module "*.vue" {
+  import type { DefineComponent } from "vue"
   const component: DefineComponent<{}, {}, any>
   export default component
 }