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/state/slides.svelte.js

@@ -1,6 +1,7 @@
 let slidesData = $state([])
 let activeIndex = $state(0)
 let pendingPath = $state(null)
+let standaloneData = $state(null)
 
 function getIndexByPath(path) {
   return slidesData.findIndex(s => s.path === path)
@@ -11,6 +12,10 @@ export const slides = {
   get activeIndex() { return activeIndex },
   get active() { return slidesData[activeIndex] ?? null },
   get pendingPath() { return pendingPath },
+  get standalone() { return standaloneData },
+
+  setStandalone(data) { standaloneData = data },
+  clearStandalone() { standaloneData = null },
 
   init(siteNavigation) {
     slidesData = siteNavigation.map(nav => ({