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/App.svelte

@@ -5,6 +5,7 @@
 
   import Header from '@components/layout/Header.svelte'
   import Cursor from '@components/layout/Cursor.svelte'
+  import Footer from '@components/layout/Footer.svelte'
   import LanguageSwitcher from '@components/ui/LanguageSwitcher.svelte'
 
   import Home from '@views/Home.svelte'
@@ -173,6 +174,8 @@
 </main>
 <LanguageSwitcher />
 
+<Footer />
+
 <style>
   :global(#app) {
     height: 100vh;