feat: collapsible section and libraries, compact card layout

- Sample Library section now collapsible via header click
- Library cards collapsible via header click (toggle icon ▾)
- Collapse All button to batch-collapse all libraries
- Compact sample cards: 28px height, 10px font, 70px min grid column
- Tighter spacing throughout (8px lib gap, 4px sample grid gap)
- Play icon removed from cards (card itself is clickable)

web/ui/index.html

@@ -40,13 +40,17 @@
   </section>
 
   <section id="wav-section">
-    <h2>Sample Library</h2>
+    <h2 class="section-toggle" data-section="wav-body">Sample Library <span class="toggle-icon">▾</span></h2>
+    <div id="wav-body">
     <div class="wav-controls">
       <input type="text" id="wav-dir" placeholder="WAV directory" value="./output/wavs">
       <button id="wav-browse-btn">Browse</button>
     </div>
     <div id="wav-results" style="display:none">
-      <div id="wav-summary" class="wav-summary"></div>
+      <div class="wav-summary-row">
+        <div id="wav-summary" class="wav-summary"></div>
+        <button id="wav-collapse-all" class="collapse-all-btn">Collapse All</button>
+      </div>
       <div id="wav-player" class="wav-player" style="display:none">
         <div id="wav-now-playing" class="now-playing"></div>
         <div class="audio-bar">
@@ -56,6 +60,7 @@
       </div>
       <div id="wav-list" class="wav-library"></div>
     </div>
+    </div>
   </section>
 
   <section id="results-section">