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/index.ts

@@ -0,0 +1,40 @@
+import Root from "./calendar.svelte";
+import Cell from "./calendar-cell.svelte";
+import Day from "./calendar-day.svelte";
+import Grid from "./calendar-grid.svelte";
+import Header from "./calendar-header.svelte";
+import Months from "./calendar-months.svelte";
+import GridRow from "./calendar-grid-row.svelte";
+import Heading from "./calendar-heading.svelte";
+import GridBody from "./calendar-grid-body.svelte";
+import GridHead from "./calendar-grid-head.svelte";
+import HeadCell from "./calendar-head-cell.svelte";
+import NextButton from "./calendar-next-button.svelte";
+import PrevButton from "./calendar-prev-button.svelte";
+import MonthSelect from "./calendar-month-select.svelte";
+import YearSelect from "./calendar-year-select.svelte";
+import Month from "./calendar-month.svelte";
+import Nav from "./calendar-nav.svelte";
+import Caption from "./calendar-caption.svelte";
+
+export {
+	Day,
+	Cell,
+	Grid,
+	Header,
+	Months,
+	GridRow,
+	Heading,
+	GridBody,
+	GridHead,
+	HeadCell,
+	NextButton,
+	PrevButton,
+	Nav,
+	Month,
+	YearSelect,
+	MonthSelect,
+	Caption,
+	//
+	Root as Calendar,
+};