perf: batch-load app status history server-side to eliminate N+1 requests

- Load all app sparkline history in a single server query
- Pass preloadedHistory to AppCard to skip client-side fetch
- Polish empty state with icon, hint text, and add button

src/lib/components/app/AppCard.svelte

@@ -20,34 +20,43 @@
 		checkedAt: string;
 	}
 
-	interface Props {
-		app: AppWithStatus;
+	interface PreloadedHistory {
+		history: StatusPoint[];
+		uptimePercent: number;
 	}
 
-	let { app }: Props = $props();
+	interface Props {
+		app: AppWithStatus;
+		preloadedHistory?: PreloadedHistory | null;
+	}
 
-	let historyData: StatusPoint[] = $state([]);
-	let uptimePercent: number | null = $state(null);
-	let historyLoading = $state(true);
+	let { app, preloadedHistory = null }: Props = $props();
+
+	let historyData: StatusPoint[] = $state(preloadedHistory?.history ?? []);
+	let uptimePercent: number | null = $state(preloadedHistory?.uptimePercent ?? null);
+	let historyLoading = $state(!preloadedHistory);
 
 	const currentStatus = $derived(app.statuses?.[0]?.status ?? 'unknown');
 
-	onMount(async () => {
-		try {
-			const res = await fetch(`/api/apps/${app.id}/history`);
-			if (res.ok) {
-				const json = await res.json();
-				if (json.success && json.data) {
-					historyData = json.data.history ?? [];
-					uptimePercent = json.data.uptimePercent ?? null;
+	// Only fetch client-side if no preloaded data was provided
+	if (!preloadedHistory) {
+		onMount(async () => {
+			try {
+				const res = await fetch(`/api/apps/${app.id}/history`);
+				if (res.ok) {
+					const json = await res.json();
+					if (json.success && json.data) {
+						historyData = json.data.history ?? [];
+						uptimePercent = json.data.uptimePercent ?? null;
+					}
 				}
+			} catch {
+				// Silently fail — sparkline is non-critical
+			} finally {
+				historyLoading = false;
 			}
-		} catch {
-			// Silently fail — sparkline is non-critical
-		} finally {
-			historyLoading = false;
-		}
-	});
+		});
+	}
 
 	const iconDisplay = $derived.by(() => {
 		if (!app.icon) return null;