feat(frontend): add dark/light theme toggle with system detection

- Add ThemeToggle component (Auto / Dark / Light) to all views
- Store preference per browser and detect system preference live
  via prefers-color-scheme
- Apply theme before first render in index.html to prevent flash
- Extend CSS with light theme variables and refactor glass styles

frontend/src/views/DashboardView.vue

@@ -4,6 +4,7 @@ import { useRoute, useRouter } from 'vue-router'
 import { useAuth } from '../stores/auth'
 import { useToast } from '../stores/toast'
 import LogoMark from '../components/LogoMark.vue'
+import ThemeToggle from '../components/ThemeToggle.vue'
 import ShareLinkCard from '../components/ShareLinkCard.vue'
 import SectionConnections from '../components/SectionConnections.vue'
 import SectionRequests from '../components/SectionRequests.vue'
@@ -62,6 +63,7 @@ function logout() {
       </div>
       <div class="row">
         <span class="muted small hide-sm">{{ auth.user.email }}</span>
+        <ThemeToggle />
         <button class="btn btn-sm" @click="logout">Abmelden</button>
       </div>
     </header>
@@ -142,7 +144,7 @@ function logout() {
   transition: transform 0.15s ease, border-color 0.15s ease;
   cursor: pointer;
 }
-.tile:hover { transform: translateY(-2px); border-color: rgba(255, 255, 255, 0.24); }
+.tile:hover { transform: translateY(-2px); border-color: var(--border-strong); }
 .tile-icon { font-size: 26px; }
 .privacy { padding: 20px 24px; }
 @media (max-width: 520px) { .hide-sm { display: none; } }