footer : fix show/hide logic, transition and scroll throttle. related to #51

- Fix scroll listener (cleanup, local querySelector, scrollHeight calc)
- Fix media query syntax in variables.css (missing space in `and (`)
- Use transform: translateY instead of bottom for GPU-accelerated transition
- Throttle scroll handler with requestAnimationFrame
- Move Footer to App.svelte (global), remove per-view imports

refs #51

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/views/Blog.svelte

@@ -5,7 +5,6 @@
 
   import Article from '@views/Article.svelte'
   import { t } from '@i18n'
-  import Footer from '@components/layout/Footer.svelte'
 
   let { data } = $props()
 
@@ -183,8 +182,6 @@
       {#if articleLoading}
         <p class="collection-loading">{t('loading')}</p>
       {/if}
-
-      <Footer />
     </div>
 
   {/if}