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/ContentRows.svelte

@@ -89,7 +89,7 @@
   ]);
 </script>
 
-<div class="content my-6 {className}">
+<div class="content mb-6 {className}">
   {#each rows as row}
     {#if row.content.length > 0}
       <div