feat: comprehensive UI/a11y/i18n fixes and pre-push quality test

- Fix ClientWrappers missing 'about' namespace (MISSING_MESSAGE error)
- Add system/light/dark theme toggle with prefers-color-scheme detection
- Rewrite 404 page with i18n, accessibility, and proper navigation
- Rewrite books page with Header/Footer, i18n, and semantic HTML
- Add i18n keys to About, Footer, and both locale files
- Fix dark mode contrast: text-stone-300/600 -> text-stone-400
- Replace raw hex bg-[#fdfcf8] with bg-stone-50 across all components
- Guard console.error in ChatWidget and manage/page behind NODE_ENV
- Add aria-label to admin login form
- Remove emoji from manage page password toggle
- Update stale dates in privacy-policy and legal-notice
- Fix ScrollFadeIn index->delay prop type error in books page
- Fix privacy-policy and legal-notice landmark structure
- Add pre-push-check.test.ts: 13-category static analysis
  (i18n parity, namespace coverage, key resolution, accessibility,
   email validation, hex colors, emojis, console guards, env docs, types)
- Add explicit i18n check step to CI workflow

messages/de.json

@@ -161,6 +161,40 @@
     "privacySettings": "Datenschutz-Einstellungen",
     "privacySettingsTitle": "Datenschutz-Banner wieder anzeigen",
     "builtWith": "Built with",
-    "aiDisclaimer": "Einige Inhalte dieser Seite können KI-generiert sein."
+    "aiDisclaimer": "Einige Inhalte dieser Seite können KI-generiert sein.",
+    "backToTop": "Nach oben",
+    "systemsOnline": "Systeme online"
+  },
+  "notFound": {
+    "title": "Seite nicht gefunden",
+    "description": "Der gesuchte Inhalt wurde verschoben, gelöscht oder hat nie existiert.",
+    "returnHome": "Zurück zur Startseite",
+    "goBack": "Zurück",
+    "exploreWork": "Arbeiten entdecken",
+    "exploreWorkDesc": "Vielleicht findest du, was du suchst, in meinem Projektarchiv?",
+    "viewProjects": "Projekte ansehen",
+    "errorReport": "Fehlerbericht"
+  },
+  "books": {
+    "title": "Bibliothek",
+    "subtitle": "Bücher, die meine Denkweise verändert und mein Wissen erweitert haben.",
+    "backHome": "Zurück",
+    "empty": "Noch keine Bücher."
+  },
+  "about": {
+    "status": "Status",
+    "aiAssistant": "KI-Assistent",
+    "library": "Bibliothek",
+    "viewAll": "Alle ansehen",
+    "myGear": "Mein Gear",
+    "gearMain": "Main",
+    "gearPC": "PC",
+    "gearServer": "Server",
+    "gearOS": "OS",
+    "curiosity": "Neugier über die Softwareentwicklung hinaus.",
+    "connect": "Verbinden",
+    "email": "E-Mail",
+    "code": "Code",
+    "professional": "Beruflich"
   }
 }

messages/en.json

@@ -164,7 +164,41 @@
     "privacySettings": "Privacy settings",
     "privacySettingsTitle": "Show privacy settings banner again",
     "builtWith": "Built with",
-    "aiDisclaimer": "Some content on this site may be AI-assisted."
+    "aiDisclaimer": "Some content on this site may be AI-assisted.",
+    "backToTop": "Back to top",
+    "systemsOnline": "Systems Online"
+  },
+  "notFound": {
+    "title": "Page not Found",
+    "description": "The content you are looking for has been moved, deleted, or never existed.",
+    "returnHome": "Return Home",
+    "goBack": "Go Back",
+    "exploreWork": "Explore Work",
+    "exploreWorkDesc": "Maybe what you need is in my project archive?",
+    "viewProjects": "View Projects",
+    "errorReport": "Error Report"
+  },
+  "books": {
+    "title": "Library",
+    "subtitle": "Books that shaped my mindset and expanded my horizons.",
+    "backHome": "Back Home",
+    "empty": "No books yet."
+  },
+  "about": {
+    "status": "Status",
+    "aiAssistant": "AI Assistant",
+    "library": "Library",
+    "viewAll": "View All",
+    "myGear": "My Gear",
+    "gearMain": "Main",
+    "gearPC": "PC",
+    "gearServer": "Server",
+    "gearOS": "OS",
+    "curiosity": "Curiosity beyond software engineering.",
+    "connect": "Connect",
+    "email": "Email",
+    "code": "Code",
+    "professional": "Professional"
   }
 }