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/templates/privacy.json.php

@@ -0,0 +1,21 @@
+<?php
+
+$bodyBlocks = [];
+foreach ($page->body()->toBlocks() as $block) {
+  if ($block->type() === 'text') {
+    $bodyBlocks[] = [
+      'type' => 'text',
+      'html' => $block->content()->text()->value(),
+    ];
+  }
+}
+
+$specificData = [
+  'backgroundImage' => $page->backgroundImage()->toFile()?->url(),
+  'body'            => $bodyBlocks,
+];
+
+$pageData = array_merge($genericData, $specificData);
+
+header('Content-Type: application/json');
+echo json_encode($pageData);