fix(ui): four small bugs surfaced after the UI/UX merge

1. Settings gear was unclickable because H1's `drop-shadow` filter
   created a stacking context that painted the H1 over the absolutely-
   positioned trigger. Bump gear wrapper to z-10.
   (src/routes/+page.svelte)

2. Switch rows in settings had a dead zone — `<label>` wrapped a
   `<span role=switch>` (not a labelable element), so taps on the
   text fell through. Move role/click/keydown to the wrapping div
   so the whole row toggles. Pill stays decorative.
   (src/lib/SettingsButton.svelte)

3. Empty cells used `dark:[filter:brightness(...)]` which creates a
   per-cell stacking context — same hazard class as bug #1. Replace
   with a pointer-events-none overlay span (`hidden dark:block`).
   (src/lib/PlayerBoard.svelte)

4. Audio cache never evicted on voice swap, leaking ~92 Audio
   elements per change over a session. New `clearAudioCache()` in
   voice.js, called from `pickVoice` when the id actually changes.
   (src/lib/voice.js, src/lib/SettingsButton.svelte)

Tests 98/98 pass; build clean.

web/src/lib/PlayerBoard.svelte

@@ -222,11 +222,19 @@
                 {@const rowComplete = hasNumber && rowCompleteness[row]}
 
                 {#if !hasNumber}
+                  <!-- Empty cell. Dark-mode dim is an overlay div, not a CSS
+                       filter, so we don't accidentally create a stacking
+                       context on every cell. -->
                   <div
                     aria-hidden="true"
-                    class="relative aspect-[3/4] sm:aspect-[3/5] border border-slate-400/50 dark:border-slate-600/40 dark:[filter:brightness(0.85)_saturate(0.9)]"
+                    class="relative aspect-[3/4] sm:aspect-[3/5] border border-slate-400/50 dark:border-slate-600/40"
                     style:background-color="var(--empty-cell-bg)"
-                  ></div>
+                  >
+                    <span
+                      aria-hidden="true"
+                      class="hidden dark:block absolute inset-0 bg-black/15 pointer-events-none"
+                    ></span>
+                  </div>
                 {:else}
                   <button
                     type="button"

web/src/lib/SettingsButton.svelte

@@ -5,6 +5,7 @@
     saveSettings,
     settings,
   } from "$lib/settings-store.svelte.js";
+  import { clearAudioCache } from "$lib/voice.js";
 
   let open = $state(false);
 
@@ -79,6 +80,7 @@
 
   /** @param {string} id */
   function pickVoice(id) {
+    if (settings.voice !== id) clearAudioCache();
     settings.voice = id;
     saveSettings();
   }
@@ -182,26 +184,28 @@
       </fieldset>
 
       {#snippet switchRow(label, isOn, onToggle)}
-        <label
+        <!-- Whole row is the switch — role/tabindex/click here, not on the
+             inner pill, so tapping the label text also toggles. -->
+        <div
+          role="switch"
+          tabindex="0"
+          aria-checked={isOn}
+          onclick={onToggle}
+          onkeydown={(/** @type {KeyboardEvent} */ e) => {
+            if (e.key === " " || e.key === "Enter") {
+              e.preventDefault();
+              onToggle();
+            }
+          }}
           class="flex items-center justify-between gap-3 px-3 py-2 rounded-lg
                  border-2 border-slate-200 dark:border-slate-600 cursor-pointer
-                 hover:border-slate-300 dark:hover:border-slate-500 transition-colors"
+                 hover:border-slate-300 dark:hover:border-slate-500 transition-colors
+                 focus:outline-none focus:ring-2 focus:ring-indigo-400"
         >
           <span class="text-sm text-slate-700 dark:text-slate-200">{label}</span>
           <span
-            role="switch"
-            tabindex="0"
-            aria-checked={isOn}
-            onclick={onToggle}
-            onkeydown={(/** @type {KeyboardEvent} */ e) => {
-              if (e.key === " " || e.key === "Enter") {
-                e.preventDefault();
-                onToggle();
-              }
-            }}
+            aria-hidden="true"
             class="relative inline-block w-10 h-6 rounded-full transition-colors flex-none
-                   focus:outline-none focus:ring-2 focus:ring-offset-2 focus:ring-indigo-400
-                   focus:ring-offset-white dark:focus:ring-offset-slate-800
                    {isOn ? 'bg-emerald-500' : 'bg-slate-300 dark:bg-slate-600'}"
           >
             <span
@@ -209,7 +213,7 @@
                      {isOn ? 'translate-x-4' : 'translate-x-0'}"
             ></span>
           </span>
-        </label>
+        </div>
       {/snippet}
 
       <!-- Master mode -->

web/src/lib/voice.js

@@ -39,6 +39,15 @@ function clipUrl(name) {
   return `${base}/audio/${settings.voice}/${name}.mp3`;
 }
 
+/**
+ * Drop all cached <audio> elements. Call when the active voice changes
+ * so the old voice's clips don't leak across a long session.
+ */
+export function clearAudioCache() {
+  cancelPlayback();
+  cache.clear();
+}
+
 export function cancelPlayback() {
   if (!isBrowser()) return;
   if (activeClip) {

web/src/routes/+page.svelte

@@ -12,7 +12,10 @@
 >
   <div class="w-full max-w-2xl">
     <header class="relative text-center mb-6">
-      <div class="absolute right-0 top-0">
+      <!-- z-10: H1 has a CSS `drop-shadow` filter which creates a stacking
+           context. Without z-index here, that filtered H1 (later in tree
+           order) paints over the gear button and swallows clicks. -->
+      <div class="absolute right-0 top-0 z-10">
         <SettingsButton />
       </div>
       <h1
@@ -23,7 +26,7 @@
       <p
         class="text-[0.65rem] sm:text-xs uppercase tracking-[0.3em] text-slate-500 dark:text-slate-400 italic mt-1"
       >
-        Hội chợ Tân Tân
+        Hội chợ TN1
       </p>
     </header>