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-select.svelte

@@ -0,0 +1,48 @@
+<script lang="ts">
+	import { Calendar as CalendarPrimitive } from "bits-ui";
+	import { cn, type WithoutChildrenOrChild } from "$lib/utils.js";
+	import ChevronDownIcon from "@lucide/svelte/icons/chevron-down";
+
+	let {
+		ref = $bindable(null),
+		class: className,
+		value,
+		onchange,
+		...restProps
+	}: WithoutChildrenOrChild<CalendarPrimitive.MonthSelectProps> = $props();
+</script>
+
+<span
+	class={cn(
+		"has-focus:border-ring border-input has-focus:ring-ring/50 relative flex rounded-md border shadow-xs has-focus:ring-[3px]",
+		className
+	)}
+>
+	<CalendarPrimitive.MonthSelect
+		bind:ref
+		class="dark:bg-popover dark:text-popover-foreground absolute inset-0 opacity-0"
+		{...restProps}
+	>
+		{#snippet child({ props, monthItems, selectedMonthItem })}
+			<select {...props} {value} {onchange}>
+				{#each monthItems as monthItem (monthItem.value)}
+					<option
+						value={monthItem.value}
+						selected={value !== undefined
+							? monthItem.value === value
+							: monthItem.value === selectedMonthItem.value}
+					>
+						{monthItem.label}
+					</option>
+				{/each}
+			</select>
+			<span
+				class="[&>svg]:text-muted-foreground flex h-8 items-center gap-1 rounded-md ps-2 pe-1 text-sm font-medium select-none [&>svg]:size-3.5"
+				aria-hidden="true"
+			>
+				{monthItems.find((item) => item.value === value)?.label || selectedMonthItem.label}
+				<ChevronDownIcon class="size-4" />
+			</span>
+		{/snippet}
+	</CalendarPrimitive.MonthSelect>
+</span>