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/index.ts

@@ -0,0 +1,19 @@
+import Root from "./popover.svelte";
+import Close from "./popover-close.svelte";
+import Content from "./popover-content.svelte";
+import Trigger from "./popover-trigger.svelte";
+import Portal from "./popover-portal.svelte";
+
+export {
+	Root,
+	Content,
+	Trigger,
+	Close,
+	Portal,
+	//
+	Root as Popover,
+	Content as PopoverContent,
+	Trigger as PopoverTrigger,
+	Close as PopoverClose,
+	Portal as PopoverPortal,
+};