Unify Further Reading into one styled box (CF-3115)

- Template renders one Further Reading aside (essential-reading style)
  with author-curated list + auto-appended layers-of-protection pillar
- Removed separate Essential reading aside
- Migrated 10 articles' ## Further Reading sections into frontmatter
  further_reading: translationKey arrays
- Added further_reading i18n key in en/de/fr

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

content/en/layers-of-online-protection-why-vpn-matters.md

@@ -7,6 +7,9 @@ categories: ["safety"]
 author: "Agiliton"
 slug: "layers-of-online-protection-why-vpn-matters"
 translationKey: "layers-of-protection"
+further_reading:
+  - "parental-controls-2026"
+  - "is-tiktok-safe"
 ---
 
 Online safety is not a switch. It is a stack of seven layers, each covering something the others don't. When parents struggle, it is almost always because they have set up one or two layers and assumed that was enough. It never is.
@@ -142,9 +145,3 @@ For a family with children under 16, run layers **2 + 3 + 4 + 7**:
 4. **In-app settings** (Layer 7) — per-app configuration for whatever your child uses
 
 Skip layers 1, 5 and 6 unless you specifically need them. They are either redundant or not load-bearing.
-
-## Further Reading
-
-- [Parental Controls in 2026: A Practical Guide](/en/parental-controls-2026-practical-guide/)
-- [Is TikTok Safe for My Child?](/en/is-tiktok-safe-for-kids/)
-- [Safe Search and Content Filtering: What Actually Works in 2026](/en/safe-search-content-filtering-2026/)