Fix toggle button styling and collapse groups by default

Toggle buttons now match the rest of the admin UI (same font, padding,
border-radius). Both start secondary/gray; the active one turns blue.
List and deck group sections are collapsed by default instead of open.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/admin.html

@@ -306,7 +306,7 @@
           </div>
           <div class="panel-col">
             <div class="toggle-btns">
-              <button type="button" id="list-new-btn">+ New list</button>
+              <button type="button" id="list-new-btn" class="secondary">+ New list</button>
               <button type="button" id="list-bulk-btn" class="secondary">⬆ Bulk import</button>
             </div>
             <div id="list-editor-section" hidden>
@@ -351,7 +351,7 @@
           </div>
           <div class="panel-col">
             <div class="toggle-btns">
-              <button type="button" id="deck-new-btn">+ New deck</button>
+              <button type="button" id="deck-new-btn" class="secondary">+ New deck</button>
               <button type="button" id="deck-bulk-btn" class="secondary">⬆ Bulk import</button>
             </div>
             <div id="deck-editor-section" hidden>