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/ui/TabBar.svelte

@@ -30,6 +30,7 @@
   role="tablist"
   aria-label={labelledBy || 'Tabs'}
   class="flex h-12 border-b border-stein-200"
+  tabindex="0"
   onkeydown={handleKeydown}
 >
   {#each tabs as tab}