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/routes/admin/articles/[id]/+page.svelte

@@ -12,6 +12,7 @@
   import { FileDropZone, MEGABYTE } from "$lib/components/ui/file-drop-zone";
   import { getAssetUrl } from "$lib/api";
   import { Select, SelectContent, SelectItem, SelectTrigger } from "$lib/components/ui/select";
+  import { DatePicker } from "$lib/components/ui/date-picker";
 
   const { data } = $props();
 
@@ -178,8 +179,8 @@
         <Input id="category" bind:value={category} />
       </div>
       <div class="space-y-1.5">
-        <Label for="publishDate">{$_("admin.common.publish_date")}</Label>
-        <Input id="publishDate" type="datetime-local" bind:value={publishDate} />
+        <Label>{$_("admin.common.publish_date")}</Label>
+        <DatePicker bind:value={publishDate} placeholder={$_("admin.common.publish_date")} />
       </div>
     </div>