Add pages, update App layout and i18n

Introduce LandingPage, LoginPage and ImprintPage components and replace the previous inline placeholders in App.jsx. Update App layout and header (branding, language toggle, dark mode toggle, route wiring) and use new i18n keys (IMPRINT and LOGIN) in en/de locales. Add client-side validation and UI for the login form. Remove unused assets/icons (public/icons.svg, src/assets/hero.png, vite.svg, vue.svg). Small logging/message tweaks and various Tailwind styling refinements.

public/i18n/de.json

@@ -1,5 +1,17 @@
 {
+  "IMPRINT": {
+    "TITLE": "Impressum",
+    "LEGAL": "Angaben gemäß § 5 DDG",
+    "CONTACT": "Kontakt"
+  },
+  "LOGIN": {
+    "TITLE": "Login",
+    "EMAIL": "Email",
+    "PASSWORD": "Passwort",
+    "SUBMIT": "Spiel betreten",
+    "ERROR_EMAIL": "Gib eine gültige Email-Adresse ein mit '@' und mit '.'.",
+    "ERROR_PASSWORD": "Das Passwort darf nicht leer sein."
+  },
   "welcome": "Dein Game-Backlog unter Kontrolle",
-  "login": "Anmelden",
-  "imprint": "Impressum"
+  "login": "Anmelden"
 }