Clickable home page stat cards; clean up nav; add buttons on list pages; move export/import to home

templates/battery_list.html

@@ -5,7 +5,8 @@
 <h1>Batteries</h1>
 
 <div class="card">
-  <div style="display:flex;justify-content:flex-end;margin-bottom:0.5rem;">
+  <div style="display:flex;justify-content:space-between;align-items:center;margin-bottom:0.5rem;">
+    <a class="btn btn-primary btn-sm" href="{{ url_for('battery_add') }}">+ Add Battery</a>
     <div style="position:relative;">
       <button type="button" id="col-picker-btn" class="btn btn-sm btn-secondary">Columns ▾</button>
       <div id="col-picker-panel" style="display:none;position:absolute;right:0;top:calc(100% + 4px);background:#fff;border:1px solid #d1d5db;border-radius:6px;padding:0.75rem 1rem;z-index:100;box-shadow:0 4px 8px rgba(0,0,0,0.1);min-width:180px;">