Fix unassign from device detail redirecting to dashboard

Pass a 'next' hidden field from the device detail unassign form so the
route redirects back to the device page instead of the dashboard.

app.py

@@ -205,7 +205,8 @@ def create_app(config_object="config"):
         battery.device_id = None
         db.commit()
         flash(f"{battery.label} unassigned and marked available.", "success")
-        return redirect(url_for("dashboard"))
+        next_url = request.form.get("next", "")
+        return redirect(next_url if next_url.startswith("/") else url_for("dashboard"))
 
     # ------------------------------------------------------------------ #
     # Battery — retire

templates/device_detail.html

@@ -95,6 +95,7 @@ function addInstallRow() {
             <td class="text-muted">{{ b.notes or '—' }}</td>
             <td>
               <form class="inline" method="post" action="{{ url_for('battery_unassign', battery_id=b.id) }}">
+                <input type="hidden" name="next" value="{{ url_for('device_detail', device_id=device.id) }}">
                 <button class="btn btn-sm btn-warning" type="submit">Unassign</button>
               </form>
             </td>