feat: replace native date inputs with shadcn date picker

Add calendar + popover components and a custom DateTimePicker wrapper.
Video forms use date-only; article forms include a time picker.
Also add video player preview to the video edit form.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/frontend/src/lib/components/ui/calendar/calendar-nav.svelte

@@ -0,0 +1,19 @@
+<script lang="ts">
+	import { cn, type WithElementRef } from "$lib/utils.js";
+	import type { HTMLAttributes } from "svelte/elements";
+
+	let {
+		ref = $bindable(null),
+		class: className,
+		children,
+		...restProps
+	}: WithElementRef<HTMLAttributes<HTMLElement>> = $props();
+</script>
+
+<nav
+	{...restProps}
+	bind:this={ref}
+	class={cn("absolute inset-x-0 top-0 flex w-full items-center justify-between gap-1", className)}
+>
+	{@render children?.()}
+</nav>