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-prev-button.svelte

@@ -0,0 +1,31 @@
+<script lang="ts">
+	import { Calendar as CalendarPrimitive } from "bits-ui";
+	import ChevronLeftIcon from "@lucide/svelte/icons/chevron-left";
+	import { buttonVariants, type ButtonVariant } from "$lib/components/ui/button/index.js";
+	import { cn } from "$lib/utils.js";
+
+	let {
+		ref = $bindable(null),
+		class: className,
+		children,
+		variant = "ghost",
+		...restProps
+	}: CalendarPrimitive.PrevButtonProps & {
+		variant?: ButtonVariant;
+	} = $props();
+</script>
+
+{#snippet Fallback()}
+	<ChevronLeftIcon class="size-4" />
+{/snippet}
+
+<CalendarPrimitive.PrevButton
+	bind:ref
+	class={cn(
+		buttonVariants({ variant }),
+		"size-(--cell-size) bg-transparent p-0 select-none disabled:opacity-50 rtl:rotate-180",
+		className
+	)}
+	children={children || Fallback}
+	{...restProps}
+/>