Take gridstack to 13, and handle the grid it may not return

gridstack 13 changed `GridStack.init` to return `GridStack | null` where
11 always handed one back. The dashboard held the result in a
`GridStack | undefined` and called `.on('change')` on it straight after,
so svelte-check stopped on two errors: null is not undefined, and the
value is possibly neither.

Coalesced to undefined so the declared type stays as it was, then
guarded before the listener is attached. `gridEl` is already checked at
the top of the function, so a null here should not occur -- but the type
allows it, and a dashboard that quietly stops persisting drags beats one
that throws inside an effect.

Nothing else in the repository touches gridstack: one import of
`GridStack` and its stylesheet, in this file.

Worth noting `vite build` passes either way. Only `npm run check` sees
this, and no CI job runs it -- the two majors would have gone in
looking clean.

web/src/routes/dashboards/[id]/+page.svelte

@@ -167,7 +167,12 @@
 	function setupGrid() {
 		if (!gridEl || !dashboard?.panels) return;
 		grid?.destroy(false);
-		grid = GridStack.init({ float: true, cellHeight: 60, column: 12 }, gridEl);
+		// gridstack 13 returns null when it cannot bind to the element, where
+		// 11 always handed back a grid. `gridEl` is guarded above so this
+		// should not happen -- but the type says it can, and a missed drag is
+		// better than a thrown error inside an effect.
+		grid = GridStack.init({ float: true, cellHeight: 60, column: 12 }, gridEl) ?? undefined;
+		if (!grid) return;
 		grid.on('change', (_event: Event, items: GridStackNode[]) => {
 			for (const item of items) {
 				const panel = dashboard?.panels?.find((p) => p.id === item.id);