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/Article.svelte

@@ -6,7 +6,6 @@
 
   import { t } from '@i18n'
   import { onMount } from 'svelte'
-  import Footer from '@components/layout/Footer.svelte'
   import WhitePaperDialog from '@components/WhitePaperDialog.svelte'
   import ShareButtons from '@components/blocks/ShareButtons.svelte'
   import ArticleRelated from '@components/blocks/ArticleRelated.svelte'
@@ -144,8 +143,6 @@
     <ArticleRelated related={data.related} />
   {/if}
 </article>
-
-<Footer />
 </div>
 
 <WhitePaperDialog uri={activeWhitePaperUri} onClose={() => activeWhitePaperUri = null} />