Three features: device dropdown filter, charge log history, unassign-all

- Device dropdowns (quick-assign, bulk install, assign page) now only show
  devices with free slots; full devices are excluded entirely
- New ChargeLog model tracks charge dates with optional cycle increment;
  battery detail page gets a Charge History card with add/delete rows
- Device list page gets per-device Unassign All button (with confirmation)
  via new POST /device/<id>/unassign-all route

templates/dashboard.html

@@ -117,7 +117,7 @@
         <select id="bulk-device-select" name="device_id"
                 style="padding:0.25rem 0.5rem;font-size:0.85rem;border:1px solid #cbd5e1;border-radius:4px;">
           <option value="">— select device —</option>
-          {% for d in devices %}
+          {% for d in devices_with_slots %}
           <option value="{{ d.id }}">{{ d.name }} ({{ d.installed_count() }}/{{ d.battery_slots }})</option>
           {% endfor %}
         </select>
@@ -181,11 +181,8 @@
                 <select id="qas-{{ b.id }}"
                         style="padding:0.2rem 0.3rem;font-size:0.8rem;border:1px solid #cbd5e1;border-radius:4px;max-width:110px;vertical-align:middle;">
                   <option value="">— assign —</option>
-                  {% for d in devices %}
-                  <option value="{{ d.id }}"
-                    {% if d.installed_count() >= d.battery_slots %}disabled{% endif %}>
-                    {{ d.name }} ({{ d.installed_count() }}/{{ d.battery_slots }})
-                  </option>
+                  {% for d in devices_with_slots %}
+                  <option value="{{ d.id }}">{{ d.name }} ({{ d.installed_count() }}/{{ d.battery_slots }})</option>
                   {% endfor %}
                 </select>
                 <button type="button" class="btn btn-sm btn-primary"