Add i18n support, language toggle UI, and translate Settings dialog (#334)

* i18n support added

* fix: align i18n implementation with Next.js 16 guide

- Rename middleware.ts to proxy.ts (Next.js 16 convention)
- Fix params type to Promise<{lang: string}> for layout/metadata
- Add 'server-only' directive and dynamic imports to dictionaries.ts
- Add hasLocale type guard and notFound() for invalid locales
- Wrap LanguageToggle in Suspense for useSearchParams
- Fix dictionary key mismatch (learnmore -> learnMore)
- Improve Chinese translations per Gemini review:
  - loading ellipsis, new -> 新建, styledMode -> 精致
  - goodResponse/badResponse -> 有帮助/无帮助
  - closeProtection -> 关闭确认, fileExceeds phrasing
- Improve Japanese translations per Gemini review:
  - closeProtection -> ページ離脱確認
  - invalidAccessCode phrasing, appendDiagram -> に追加
  - styledMode -> スタイル付き

---------

Co-authored-by: dayuan.jiang <jdy.toh@gmail.com>

package.json

@@ -22,6 +22,7 @@
         "@ai-sdk/openai": "^2.0.19",
         "@ai-sdk/react": "^2.0.107",
         "@aws-sdk/credential-providers": "^3.943.0",
+        "@formatjs/intl-localematcher": "^0.7.2",
         "@langfuse/client": "^4.4.9",
         "@langfuse/otel": "^4.4.4",
         "@langfuse/tracing": "^4.4.9",
@@ -48,6 +49,7 @@
         "jsonrepair": "^3.13.1",
         "lucide-react": "^0.483.0",
         "motion": "^12.23.25",
+        "negotiator": "^1.0.0",
         "next": "^16.0.7",
         "ollama-ai-provider-v2": "^1.5.4",
         "pako": "^2.1.0",
@@ -59,6 +61,7 @@
         "react-markdown": "^10.1.0",
         "react-resizable-panels": "^3.0.6",
         "remark-gfm": "^4.0.1",
+        "server-only": "^0.0.1",
         "sonner": "^2.0.7",
         "tailwind-merge": "^3.0.2",
         "tailwindcss-animate": "^1.0.7",
@@ -76,6 +79,7 @@
         "@biomejs/biome": "^2.3.8",
         "@tailwindcss/postcss": "^4",
         "@tailwindcss/typography": "^0.5.19",
+        "@types/negotiator": "^0.6.4",
         "@types/node": "^20",
         "@types/pako": "^2.0.3",
         "@types/react": "^19",