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