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/Toggle.svelte

@@ -21,7 +21,7 @@
     />
     <span
       class="absolute top-0.5 inline-block h-5 w-5 rounded-full bg-white shadow-sm transition-transform duration-200 {checked ? 'left-[22px] translate-x-0.5' : 'left-0.5'}"
-    />
+    ></span>
   </span>
   {#if label}
     <span class="text-base text-stein-700">{label}</span>