feat(i18n): add i18next foundation with English + zh-CN locales

Installs i18next + react-i18next + language detector and wires up a
language selector in the General settings page. Extracts strings from
the highest-visibility surfaces: all settings tabs, model management,
sidebar nav, main editor, and the floating generate box. Remaining
strings (profile forms, history, stories/effects/voices/audio tabs)
can land in follow-up PRs.

Closes #411.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

app/src/components/ServerTab/LanguageSelect.tsx

@@ -0,0 +1,29 @@
+import { useTranslation } from 'react-i18next';
+import {
+  Select,
+  SelectContent,
+  SelectItem,
+  SelectTrigger,
+  SelectValue,
+} from '@/components/ui/select';
+import { SUPPORTED_LANGUAGES } from '@/i18n';
+
+export function LanguageSelect() {
+  const { i18n } = useTranslation();
+  const current = SUPPORTED_LANGUAGES.find((l) => l.code === i18n.resolvedLanguage)?.code ?? 'en';
+
+  return (
+    <Select value={current} onValueChange={(value) => i18n.changeLanguage(value)}>
+      <SelectTrigger className="h-9 w-[180px]">
+        <SelectValue />
+      </SelectTrigger>
+      <SelectContent>
+        {SUPPORTED_LANGUAGES.map((lang) => (
+          <SelectItem key={lang.code} value={lang.code}>
+            {lang.label}
+          </SelectItem>
+        ))}
+      </SelectContent>
+    </Select>
+  );
+}