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-head-cell.svelte

@@ -0,0 +1,19 @@
+<script lang="ts">
+	import { Calendar as CalendarPrimitive } from "bits-ui";
+	import { cn } from "$lib/utils.js";
+
+	let {
+		ref = $bindable(null),
+		class: className,
+		...restProps
+	}: CalendarPrimitive.HeadCellProps = $props();
+</script>
+
+<CalendarPrimitive.HeadCell
+	bind:ref
+	class={cn(
+		"text-muted-foreground w-(--cell-size) rounded-md text-[0.8rem] font-normal",
+		className
+	)}
+	{...restProps}
+/>