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-month.svelte

@@ -0,0 +1,15 @@
+<script lang="ts">
+	import { type WithElementRef, cn } from "$lib/utils.js";
+	import type { HTMLAttributes } from "svelte/elements";
+
+	let {
+		ref = $bindable(null),
+		class: className,
+		children,
+		...restProps
+	}: WithElementRef<HTMLAttributes<HTMLElement>> = $props();
+</script>
+
+<div {...restProps} bind:this={ref} class={cn("flex flex-col", className)}>
+	{@render children?.()}
+</div>