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>

hooks/use-dictionary.ts

@@ -0,0 +1,29 @@
+"use client"
+
+import React, { createContext, useContext } from "react"
+import type { Dictionary } from "@/lib/i18n/dictionaries"
+
+const DictionaryContext = createContext<Dictionary | null>(null)
+
+export function DictionaryProvider({
+    children,
+    dictionary,
+}: React.PropsWithChildren<{ dictionary: Dictionary }>) {
+    return React.createElement(
+        DictionaryContext.Provider,
+        { value: dictionary },
+        children,
+    )
+}
+
+export function useDictionary() {
+    const dict = useContext(DictionaryContext)
+    if (!dict) {
+        throw new Error(
+            "useDictionary must be used within a DictionaryProvider",
+        )
+    }
+    return dict
+}
+
+export default useDictionary