Fix: dialog scroll, isDirty false positive, mobile edit btn, widget copy

Five fixes:

1. Dialog mobile scroll: DialogContent now has max-h-[calc(100dvh-2rem)]
   overflow-y-auto so dialogs that don't fit on screen can scroll
   instead of clipping their footer (and Cancel button) off-screen.

2. isDirty false positive: WidgetConfigDialog's SheetForm used
   isDirty={draft !== null} which was true the moment you opened edit
   mode, even with no changes. Now stores a draftBaseline at startEdit
   time and compares JSON.stringify(draft) !== JSON.stringify(baseline).
   The discard-confirmation only appears when something actually changed.

3. Mobile edit button always visible: the widget card's edit button was
   opacity-0 group-hover:opacity-100 (hover-only). Changed to
   md:opacity-0 md:group-hover:opacity-100 — always visible below md,
   hover-reveal at md+.

4. Copy button for referenced widgets: WidgetInstanceCard gains an onCopy
   prop. On the Dashboard, referenced widgets get a Copy icon button that
   triggers detachRef (creates an independent clone). The edit button on
   referenced widgets edits the original (shared config).

5. ConfirmDialog Cancel: fixed by #1 (the Cancel button was off-screen
   on mobile dialogs that couldn't scroll).

128 tests pass; lint/build green.

frontend/src/pages/service-tabs/OverviewTab.tsx

@@ -49,7 +49,14 @@ export function OverviewTab({ instance }: { instance: ServiceInstance }) {
 			{visibleWidgets.length > 0 ? (
 				<div className="grid grid-cols-1 gap-4 md:grid-cols-2">
 					{visibleWidgets.map((widget) => (
-						<WidgetInstanceCard key={widget.id} widget={widget} onEdit={(id) => { setEditWidgetId(id); setConfigOpen(true); }} />
+						<WidgetInstanceCard
+							key={widget.id}
+							widget={widget}
+							onEdit={(id) => {
+								setEditWidgetId(id);
+								setConfigOpen(true);
+							}}
+						/>
 					))}
 				</div>
 			) : (
@@ -72,7 +79,10 @@ export function OverviewTab({ instance }: { instance: ServiceInstance }) {
 
 			<WidgetConfigDialog
 				open={configOpen}
-				onClose={() => { setConfigOpen(false); setEditWidgetId(undefined); }}
+				onClose={() => {
+					setConfigOpen(false);
+					setEditWidgetId(undefined);
+				}}
 				serviceId={instance.id}
 				editWidgetId={editWidgetId}
 			/>