DevbarPanels class

Serves a panel for as long as the handle is held.

The imperative half of AddDevbarPanel, the same way UiService.addButton is the imperative half of AddDevbarButton. The widget is the wrapper: it calls this in initState and removes in dispose.

_panel = DevbarPanels.add(DatabasePanelSource(adapter));  // at login
_panel.remove();                                          // at logout

Which half to use is a question about the scope, not about taste. A panel belonging to a subtree — a signed-in shell, a checkout flow — should be declared where that subtree is, and the widget makes forgetting to remove it impossible. A panel belonging to a service — a session opened at login and closed at logout, an environment being switched — has no subtree to hang from, and reaching for the widget there means inventing a widget boundary for a lifetime that is not one. Inserting a widget above an existing subtree remounts it.

Needs no Devbar above it, and no BuildContext: a panel is served to flutterware, not drawn in the overlay. Outside flutterware add hands back an inert handle rather than null, so the call site never grows a ?.