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/components/ThemeToggle.vue

@@ -0,0 +1,30 @@
+<script setup lang="ts">
+import { useTheme } from '../lib/theme'
+
+const { mode, labels, cycle } = useTheme()
+const icons = { auto: '🌗', dark: '🌙', light: '☀️' } as const
+</script>
+
+<template>
+  <button
+    class="btn btn-sm theme-toggle"
+    type="button"
+    :title="`Design: ${labels[mode]} – klicken zum Wechseln`"
+    :aria-label="`Design umschalten, aktuell ${labels[mode]}`"
+    @click="cycle"
+  >
+    <span aria-hidden="true">{{ icons[mode] }}</span>
+    <span class="tlabel">{{ labels[mode] }}</span>
+  </button>
+</template>
+
+<style scoped>
+.tlabel {
+  margin-left: 7px;
+}
+@media (max-width: 600px) {
+  .tlabel {
+    display: none;
+  }
+}
+</style>