Modernize frontend: new design system, redesign all pages

- Convert codexInfo.md to CLAUDE.md as the central project context.
- Rewrite GUI/style.md with a modernized, file-first direction (layered
  surfaces, refined typography, motion budget, ambient gradients).
- Refresh global tokens in global.css, page-layouts.css and
  surface-patterns.css: extended palette (primary 050/800, surface
  elevated, border subtle), four-tier shadow system, dark-mode parity,
  new utilities (hoard-chip, hoard-icon-tile, hoard-spotlight,
  hoard-section-head, hoard-divider-soft, status pulse dot).
- Rebuild Layout.vue: premium app shell with brand halo, animated
  active-indicator, account pill with avatar/initials, drawer footer
  card, refined banner stack and footer.
- Redesign every route while preserving routing and API contracts:
  Home, Login, ChangePassword, Dashboard, AdminUsers, AdminUserDetail,
  Impressum, 404, Forbidden. Adds search/admin stats, password hint
  list, dashboard greeting with avatar, modernized hero/spotlight
  treatments and consistent mobile layouts (safe-areas, 44/48px tap
  targets).
- Drop @fontsource/roboto import in vuetify.ts and load Inter via the
  rsms.me CSS in index.html; update Vuetify defaults and palette to
  match the new tokens.

GUI/src/routes/Forbidden.vue

@@ -8,6 +8,9 @@ const isLoading = ref(true)
 const isAuthenticated = ref(false)
 
 const primaryActionLabel = computed(() => (isAuthenticated.value ? 'Zum Dashboard' : 'Zum Login'))
+const primaryActionIcon = computed(() =>
+  isAuthenticated.value ? 'mdi-view-dashboard-outline' : 'mdi-login',
+)
 
 async function resolveAuthState() {
   try {
@@ -29,6 +32,15 @@ async function navigatePrimaryAction() {
   await router.replace({ name: 'Login' })
 }
 
+async function navigateBack() {
+  if (window.history.length > 1) {
+    router.back()
+    return
+  }
+
+  await router.replace({ name: isAuthenticated.value ? 'Dashboard' : 'Home' })
+}
+
 onMounted(() => {
   void resolveAuthState()
 })
@@ -36,23 +48,34 @@ onMounted(() => {
 
 <template>
   <v-container fluid class="forbidden-page hoard-page hoard-page--centered">
-    <section class="forbidden-shell hoard-shell-grid hoard-panel">
+    <section class="forbidden-shell hoard-panel hoard-panel-gradient hoard-spotlight">
+      <div class="forbidden-icon">
+        <span class="forbidden-icon__halo" aria-hidden="true" />
+        <v-icon icon="mdi-shield-alert-outline" size="40" />
+      </div>
+
       <header class="forbidden-head">
-        <p class="hoard-kicker">Fehlende Berechtigung</p>
-        <h1>Kein Zugriff</h1>
-        <p>Dein Konto hat keine ausreichende Rolle für diese Seite.</p>
+        <p class="hoard-kicker hoard-kicker--wide">Fehlende Berechtigung</p>
+        <h1>Kein Zugriff.</h1>
+        <p>
+          Dein Konto hat aktuell keine ausreichende Rolle, um diese Seite zu sehen. Falls das ein Fehler ist,
+          wende dich an einen Admin – oder wechsle zurück in deinen freigegebenen Bereich.
+        </p>
       </header>
 
       <div class="forbidden-actions hoard-action-row">
         <v-btn
-          color="primary"
-          prepend-icon="mdi-arrow-right"
+          variant="elevated"
+          :prepend-icon="primaryActionIcon"
           :loading="isLoading"
           :disabled="isLoading"
           @click="navigatePrimaryAction"
         >
           {{ primaryActionLabel }}
         </v-btn>
+        <v-btn variant="outlined" prepend-icon="mdi-arrow-left" @click="navigateBack">
+          Zurück
+        </v-btn>
       </div>
     </section>
   </v-container>
@@ -60,30 +83,91 @@ onMounted(() => {
 
 <style scoped>
 .forbidden-page {
-  --hoard-shell-width: min(640px, 100%);
+  --hoard-centered-offset: 200px;
 }
 
 .forbidden-shell {
-  gap: var(--space-4);
+  --hoard-gradient-angle: 130deg;
+  --hoard-gradient-start: color-mix(in srgb, var(--color-warning) 14%, var(--color-surface) 86%);
+  --hoard-gradient-end: var(--color-surface);
+  --hoard-gradient-end-stop: 65%;
+
+  display: flex;
+  flex-direction: column;
+  gap: var(--space-5);
+  align-items: center;
+  text-align: center;
+  width: min(640px, 100%);
+  padding: var(--space-10) var(--space-7);
+  border-radius: var(--radius-xl);
 }
 
-.forbidden-head h1,
-.forbidden-head p {
-  margin: 0;
+.forbidden-icon {
+  position: relative;
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  width: 80px;
+  height: 80px;
+  border-radius: var(--radius-full);
+  background:
+    linear-gradient(135deg, var(--color-warning), color-mix(in srgb, var(--color-warning) 60%, var(--color-danger) 40%));
+  color: #fff;
+  box-shadow:
+    var(--shadow-md),
+    inset 0 0 0 2px color-mix(in srgb, var(--color-accent-lime) 18%, transparent);
+}
+
+.forbidden-icon__halo {
+  position: absolute;
+  inset: -10px;
+  border-radius: var(--radius-full);
+  background:
+    radial-gradient(
+      closest-side,
+      color-mix(in srgb, var(--color-warning) 38%, transparent),
+      transparent 70%
+    );
+  filter: blur(12px);
+  opacity: 0.6;
 }
 
 .forbidden-head h1 {
-  margin-top: var(--space-2);
-  margin-bottom: var(--space-2);
+  margin: 0 0 var(--space-2);
+  font-size: clamp(1.8rem, 1.4rem + 1vw, 2.4rem);
+  font-weight: 700;
+  letter-spacing: -0.02em;
 }
 
 .forbidden-head p {
+  margin: 0 auto;
+  max-width: 50ch;
   color: var(--color-text-secondary);
+  line-height: 1.6;
+}
+
+.forbidden-actions {
+  justify-content: center;
 }
 
 @media (prefers-reduced-motion: no-preference) {
   .forbidden-shell {
-    animation: hoard-soft-enter 260ms both;
+    animation: hoard-soft-enter 280ms both;
+  }
+}
+
+@media (width <= 600px) {
+  .forbidden-shell {
+    padding: var(--space-7) var(--space-5);
+  }
+
+  .forbidden-actions {
+    width: 100%;
+  }
+
+  :deep(.forbidden-actions .v-btn) {
+    width: 100%;
+    min-height: 44px;
   }
 }
 </style>