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/package.json

@@ -15,7 +15,7 @@
     "@iconify-json/ri": "^1.2.10",
     "@iconify/tailwind4": "^1.2.1",
     "@internationalized/date": "^3.11.0",
-    "@lucide/svelte": "^0.577.0",
+    "@lucide/svelte": "^0.561.0",
     "@sveltejs/adapter-node": "^5.5.4",
     "@sveltejs/adapter-static": "^3.0.10",
     "@sveltejs/kit": "^2.53.4",