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/Button.svelte

@@ -9,6 +9,7 @@
     loading = false,
     type = 'button' as 'button' | 'submit' | 'reset',
     label = '',
+    children,
   } = $props();
 
   const base =
@@ -40,7 +41,9 @@
   {#if loading}
     <span class="inline-block h-4 w-4 animate-spin rounded-full border-2 border-current border-t-transparent"></span>
     <span>Laden...</span>
+  {:else if children}
+    {@render children()}
   {:else}
-    <slot>{label}</slot>
+    {label}
   {/if}
 </button>