fix: update styling and structure in various components

- Changed margin class from `my-6` to `mb-6` in ContentRows.svelte for consistent spacing.
- Updated PostCard.svelte to use `$derived` for post properties, enhancing reactivity.
- Refined DeadlineBannerBlock.svelte by removing unnecessary border classes and improving layout for better content alignment.
- Modified Button.svelte to support rendering children directly, enhancing flexibility.
- Added `tabindex="0"` to TabBar.svelte for improved accessibility.
- Fixed textarea closing tag in Textarea.svelte for proper HTML structure.
- Corrected span closing tag in Toggle.svelte for valid markup.

src/lib/components/PostCard.svelte

@@ -29,7 +29,7 @@
     eventDate?: string;
     eventLocation?: { text?: string };
   };
-  const p = post as PostWithEvent;
+  const p = $derived(post as PostWithEvent);
 
   const eventDate = $derived(p.isEvent && p.eventDate ? p.eventDate : null);
   const locationText = $derived(p.eventLocation?.text ?? null);