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/battery_detail.html

@@ -148,6 +148,66 @@
   </form>
 </div>
 
+<!-- Charge History -->
+<div class="card">
+  <h2>Charge History</h2>
+
+  {% if charge_logs %}
+  <div class="table-wrap">
+    <table class="responsive-table">
+      <thead>
+        <tr>
+          <th>Date</th>
+          <th>+Cycle</th>
+          <th>Notes</th>
+          <th></th>
+        </tr>
+      </thead>
+      <tbody>
+        {% for log in charge_logs %}
+        <tr>
+          <td data-label="Date">{{ log.charged_date }}</td>
+          <td data-label="+Cycle">{{ '✓' if log.increment_cycles else '—' }}</td>
+          <td data-label="Notes" class="text-muted">{{ log.notes or '—' }}</td>
+          <td data-label="">
+            <form class="inline" method="post"
+                  action="{{ url_for('battery_charge_log_delete', battery_id=battery.id, log_id=log.id) }}"
+                  onsubmit="return confirm('Delete this charge log entry?')">
+              <button class="btn btn-sm btn-danger" type="submit">Delete</button>
+            </form>
+          </td>
+        </tr>
+        {% endfor %}
+      </tbody>
+    </table>
+  </div>
+  {% else %}
+  <p class="text-muted" style="margin-bottom:0.75rem;">No charge log entries yet.</p>
+  {% endif %}
+
+  <h3 style="font-size:1rem;margin:1rem 0 0.5rem;color:var(--text-h2);">Add Charge Entry</h3>
+  <form method="post" action="{{ url_for('battery_charge_log_add', battery_id=battery.id) }}"
+        style="display:flex;gap:0.5rem;flex-wrap:wrap;align-items:flex-end;">
+    <div class="form-group" style="margin:0;flex:1;min-width:140px;">
+      <label>Date</label>
+      <input type="date" name="charged_date" required>
+    </div>
+    <div class="form-group" style="margin:0;align-self:flex-end;padding-bottom:1rem;">
+      <label style="display:flex;align-items:center;gap:0.4rem;font-weight:normal;cursor:pointer;">
+        <input type="checkbox" name="increment_cycles" value="1" checked>
+        Increment charge cycles
+      </label>
+    </div>
+    <div class="form-group" style="margin:0;flex:2;min-width:160px;">
+      <label>Notes (optional)</label>
+      <input type="text" name="notes" placeholder="e.g. trickle charge overnight">
+    </div>
+    <div style="padding-bottom:1rem;">
+      <button class="btn btn-primary" type="submit">Add</button>
+    </div>
+  </form>
+</div>
+
 <!-- Edit Details -->
 <div class="card">
   <h2>Edit Details</h2>