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>

proxy.ts

@@ -0,0 +1,63 @@
+import { match as matchLocale } from "@formatjs/intl-localematcher"
+import Negotiator from "negotiator"
+import type { NextRequest } from "next/server"
+import { NextResponse } from "next/server"
+import { i18n } from "./lib/i18n/config"
+
+function getLocale(request: NextRequest): string | undefined {
+    // Negotiator expects plain object so we need to transform headers
+    const negotiatorHeaders: Record<string, string> = {}
+    request.headers.forEach((value, key) => {
+        negotiatorHeaders[key] = value
+    })
+
+    // @ts-expect-error locales are readonly
+    const locales: string[] = i18n.locales
+
+    // Use negotiator and intl-localematcher to get best locale
+    const languages = new Negotiator({ headers: negotiatorHeaders }).languages(
+        locales,
+    )
+
+    const locale = matchLocale(languages, locales, i18n.defaultLocale)
+
+    return locale
+}
+
+export function proxy(request: NextRequest) {
+    const pathname = request.nextUrl.pathname
+
+    // Skip API routes, static files, and Next.js internals
+    if (
+        pathname.startsWith("/api/") ||
+        pathname.startsWith("/_next/") ||
+        pathname.includes("/favicon") ||
+        /\.(.*)$/.test(pathname)
+    ) {
+        return
+    }
+
+    // Check if there is any supported locale in the pathname
+    const pathnameIsMissingLocale = i18n.locales.every(
+        (locale) =>
+            !pathname.startsWith(`/${locale}/`) && pathname !== `/${locale}`,
+    )
+
+    // Redirect if there is no locale
+    if (pathnameIsMissingLocale) {
+        const locale = getLocale(request)
+
+        // Redirect to localized path
+        return NextResponse.redirect(
+            new URL(
+                `/${locale}${pathname.startsWith("/") ? "" : "/"}${pathname}`,
+                request.url,
+            ),
+        )
+    }
+}
+
+export const config = {
+    // Matcher ignoring `/_next/` and `/api/`
+    matcher: ["/((?!api|_next/static|_next/image|favicon.ico).*)"],
+}