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/en.json

@@ -1,5 +1,17 @@
 {
+  "IMPRINT": {
+    "TITLE": "Imprint",
+    "LEGAL": "Information according to § 5 TMG.",
+    "CONTACT": "Contact"
+  },
+  "LOGIN": {
+    "TITLE": "Login",
+    "EMAIL": "Email",
+    "PASSWORD": "Password",
+    "SUBMIT": "Enter the game",
+    "ERROR_EMAIL": "Enter a valid email address with '@' and with '.'.",
+    "ERROR_PASSWORD": "The password must have at least one character."
+  },
   "welcome": "Control your game backlog",
-  "login": "Login",
-  "imprint": "Imprint"
+  "login": "Login"
 }