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/FontChange.vue

@@ -50,22 +50,14 @@ const fontSize = computed({
   <div class="font-change">
     <div>
       <label for="title-font" class="font-label">t</label>
-      <select
-        id="title-font"
-        class="select"
-        v-model="titleFont"
-      >
+      <select id="title-font" class="select" v-model="titleFont">
         <option v-for="font in sortedFontFamilies" :key="font" :value="font">
           {{ font }}
         </option>
       </select>
 
       <label for="body-font" class="font-label">p</label>
-      <select
-        id="body-font"
-        class="select"
-        v-model="bodyFont"
-      >
+      <select id="body-font" class="select" v-model="bodyFont">
         <option v-for="font in sortedFontFamilies" :key="font" :value="font">
           {{ font }}
         </option>
@@ -75,11 +67,7 @@ const fontSize = computed({
       <theme-swap />
 
       <label for="font-size" class="font-label">s</label>
-      <select
-        id="font-size"
-        class="select"
-        v-model="fontSize"
-      >
+      <select id="font-size" class="select" v-model="fontSize">
         <option v-for="size in fontSizes" :key="size" :value="size">
           {{ size }}
         </option>