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>

site/blueprints/pages/privacy.yml

@@ -0,0 +1,41 @@
+title: Confidentialité
+icon: lock
+
+tabs:
+  content:
+    label: Contenu
+    icon: text
+    columns:
+      - width: 1/3
+        fields:
+          backgroundImage:
+            label: Image de fond
+            type: files
+            max: 1
+            layout: cards
+            accept: image/*
+            translate: false
+
+      - width: 2/3
+        fields:
+          body:
+            label: Corps
+            type: blocks
+            fieldsets:
+              text:
+                extends: blocks/text
+                fields:
+                  text:
+                    headings:
+                      - 3
+                    nodes:
+                      - heading
+                    marks:
+                      - bold
+                      - italic
+                      - underline
+                      - strike
+                      - link
+
+  files: tabs/files
+  seo: seo/page