feat: add 6 new locales (ja, ko, fr, es, de, pt) and UI polish

- Add Japanese, Korean, French, Spanish, German, Portuguese translations
- Improve session active state visibility in both themes
- Static language labels in LanguageSwitch component
- Dark theme: lighten chat input background for better contrast
- Fix system theme listener not toggling back to light

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/client/src/components/layout/LanguageSwitch.vue

@@ -1,16 +1,19 @@
 <script setup lang="ts">
-import { computed } from 'vue'
 import { useI18n } from 'vue-i18n'
 import { NSelect } from 'naive-ui'
 
-const { locale, availableLocales, t } = useI18n()
+const { locale } = useI18n()
 
-const options = computed(() =>
-  availableLocales.map(loc => ({
-    label: loc === 'zh' ? t('language.zh') : t('language.en'),
-    value: loc,
-  })),
-)
+const options = [
+  { label: '中文', value: 'zh' },
+  { label: 'English', value: 'en' },
+  { label: '日本語', value: 'ja' },
+  { label: '한국어', value: 'ko' },
+  { label: 'Français', value: 'fr' },
+  { label: 'Español', value: 'es' },
+  { label: 'Deutsch', value: 'de' },
+  { label: 'Português', value: 'pt' },
+]
 
 function handleChange(val: string) {
   locale.value = val