feat: add dark theme support with CSS custom properties and Naive UI integration

Implement runtime theme switching using CSS custom properties delegated through SCSS variables, with light/dark/system modes, FOUC prevention, sidebar toggle, and settings selector. Add theme-aware video assets for sidebar and chat thinking indicator.

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

packages/client/src/components/hermes/settings/DisplaySettings.vue

@@ -1,12 +1,20 @@
 <script setup lang="ts">
-import { NSwitch, useMessage } from 'naive-ui'
+import { NSwitch, NSelect, useMessage } from 'naive-ui'
 import { useI18n } from 'vue-i18n'
 import { useSettingsStore } from '@/stores/hermes/settings'
+import { useTheme, type ThemeMode } from '@/composables/useTheme'
 import SettingRow from './SettingRow.vue'
 
 const settingsStore = useSettingsStore()
 const message = useMessage()
 const { t } = useI18n()
+const { mode, setMode } = useTheme()
+
+const themeOptions = [
+  { label: t('settings.display.themeLight'), value: 'light' },
+  { label: t('settings.display.themeDark'), value: 'dark' },
+  { label: t('settings.display.themeSystem'), value: 'system' },
+]
 
 async function save(values: Record<string, any>) {
   try {
@@ -16,10 +24,19 @@ async function save(values: Record<string, any>) {
     message.error(t('settings.saveFailed'))
   }
 }
+
+function handleThemeChange(val: string) {
+  const m = val as ThemeMode
+  setMode(m)
+  save({ skin: m })
+}
 </script>
 
 <template>
   <section class="settings-section">
+    <SettingRow :label="t('settings.display.theme')" :hint="t('settings.display.themeHint')">
+      <NSelect :value="mode" :options="themeOptions" size="small" :consistent-menu-width="false" class="input-sm" @update:value="handleThemeChange" />
+    </SettingRow>
     <SettingRow :label="t('settings.display.streaming')" :hint="t('settings.display.streamingHint')">
       <NSwitch :value="settingsStore.display.streaming" @update:value="v => save({ streaming: v })" />
     </SettingRow>

packages/client/src/components/hermes/settings/PlatformCard.vue

@@ -57,7 +57,7 @@ const configured = computed(() => {
   overflow: hidden;
 
   &.configured {
-    border-color: rgba($success, 0.2);
+    border-color: rgba(var(--success-rgb), 0.2);
   }
 }
 
@@ -70,7 +70,7 @@ const configured = computed(() => {
   user-select: none;
 
   &:hover {
-    background-color: rgba($text-primary, 0.03);
+    background-color: rgba(var(--text-primary-rgb), 0.03);
   }
 }