feat: replace all vanilla HTML elements with shadcn components

- Install skeleton and tooltip shadcn components
- event-dialog: textarea → Textarea, duration chips → Button ghost
- date-time-picker: raw button trigger → Button outline, quick-date chips → Button ghost
- event-card: <a> URL link → Button link asChild
- sign-in: animate-pulse div → Skeleton
- ai-toolbar: animate-pulse div → Skeleton, event count span → Badge secondary
- event-actions-toolbar: event count span → Badge secondary

src/components/ui/skeleton.tsx

@@ -0,0 +1,13 @@
+import { cn } from "@/lib/utils";
+
+function Skeleton({ className, ...props }: React.ComponentProps<"div">) {
+	return (
+		<div
+			data-slot="skeleton"
+			className={cn("animate-pulse rounded-md bg-accent", className)}
+			{...props}
+		/>
+	);
+}
+
+export { Skeleton };