Add device_type field, mobile-friendly improvements, and device filtering

- Device model: add device_type column (String 50, nullable)
- Device add/edit: type select with presets + custom entry
- Device detail: show type in info card; new Edit Device form
- Device list: Type column + client-side filter bar (type + text search)
- Mobile: card-style responsive tables on dashboard and device list,
  form-grid-2col collapse, larger tap targets, stacked form-actions,
  column picker viewport fix, filter bar full-width controls
- Assign page: larger radio touch targets (min-height 44px)
- 3 new acceptance tests for device_type (45 total)

templates/battery_add.html

@@ -7,7 +7,7 @@
 <div class="card">
   <form method="post" action="{{ url_for('battery_add') }}">
 
-    <div style="display:grid;grid-template-columns:1fr 1fr;gap:0 1rem;">
+    <div class="form-grid-2col" style="display:grid;grid-template-columns:1fr 1fr;gap:0 1rem;">
 
       <div class="form-group" style="grid-column:1/-1;">
         <label for="brand-select">Brand <span class="text-danger">*</span></label>