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/popover/popover-portal.svelte

@@ -0,0 +1,7 @@
+<script lang="ts">
+	import { Popover as PopoverPrimitive } from "bits-ui";
+
+	let { ...restProps }: PopoverPrimitive.PortalProps = $props();
+</script>
+
+<PopoverPrimitive.Portal {...restProps} />