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/theme.config.ts

@@ -2,6 +2,6 @@
 // Update these values to change the light and dark themes
 
 export const themeConfig = {
-  light: 'emerald',
-  dark: 'forest'
+  light: "emerald",
+  dark: "forest"
 }