add privacy page as standalone SPA view outside slide navigation

- New Kirby template/blueprint/JSON for privacy page (confidentialite slug)
- Standalone page state in slides store + router handling for non-nav pages
- Privacy.svelte view with background image, text blocks, footer
- Centralize vertical lines in App.svelte as fixed elements with per-slide visibility
- Footer privacy link language-aware (FR/EN)
- Portfolio mockup fix: read from default language for consistent EN display
- menu.php: add privacy page to Kirby panel navigation

refs #44

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

src/components/layout/Footer.svelte

@@ -1,5 +1,6 @@
 <script>
   import { site } from '@state/site.svelte'
+  import { locale } from '@state/locale.svelte'
   import { t } from '@i18n'
 
   const logo    = $derived(site.logo)
@@ -89,9 +90,9 @@
     <div class="footer-divider" aria-hidden="true"></div>
     {#if contact.legalNotice}
       <a href={contact.legalNotice} target="_blank" rel="noopener noreferrer">{t('legal')}</a>
-    {:else}
-      <a href="/privacy">{t('privacy')}</a>
     {/if}
+    <div class="footer-divider" aria-hidden="true"></div>
+    <a href={locale.current === 'en' ? '/en/confidentialite' : '/confidentialite'}>{t('privacy')}</a>
   </div>
 </footer>