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>

components/language-toggle.tsx

@@ -0,0 +1,108 @@
+"use client"
+
+import { Globe } from "lucide-react"
+import { usePathname, useRouter, useSearchParams } from "next/navigation"
+import { Suspense, useEffect, useRef, useState } from "react"
+import { i18n, type Locale } from "@/lib/i18n/config"
+
+const LABELS: Record<string, string> = {
+    en: "EN",
+    zh: "中文",
+    ja: "日本語",
+}
+
+function LanguageToggleInner({ className = "" }: { className?: string }) {
+    const router = useRouter()
+    const pathname = usePathname() || "/"
+    const search = useSearchParams()
+    const [open, setOpen] = useState(false)
+    const [value, setValue] = useState<Locale>(i18n.defaultLocale)
+    const ref = useRef<HTMLDivElement | null>(null)
+
+    useEffect(() => {
+        const seg = pathname.split("/").filter(Boolean)
+        const first = seg[0]
+        if (first && i18n.locales.includes(first as Locale))
+            setValue(first as Locale)
+        else setValue(i18n.defaultLocale)
+    }, [pathname])
+
+    useEffect(() => {
+        function onDoc(e: MouseEvent) {
+            if (!ref.current) return
+            if (!ref.current.contains(e.target as Node)) setOpen(false)
+        }
+        if (open) document.addEventListener("mousedown", onDoc)
+        return () => document.removeEventListener("mousedown", onDoc)
+    }, [open])
+
+    const changeLocale = (lang: string) => {
+        const parts = pathname.split("/")
+        if (parts.length > 1 && i18n.locales.includes(parts[1] as Locale)) {
+            parts[1] = lang
+        } else {
+            parts.splice(1, 0, lang)
+        }
+        const newPath = parts.join("/") || "/"
+        const searchStr = search?.toString() ? `?${search.toString()}` : ""
+        setOpen(false)
+        router.push(newPath + searchStr)
+    }
+
+    return (
+        <div className={`relative inline-flex ${className}`} ref={ref}>
+            <button
+                aria-haspopup="menu"
+                aria-expanded={open}
+                onClick={() => setOpen((s) => !s)}
+                className="p-2 rounded-full hover:bg-accent/20 transition-colors text-muted-foreground"
+                aria-label="Change language"
+            >
+                <Globe className="w-5 h-5" />
+            </button>
+            {open && (
+                <div className="absolute right-0 top-full mt-2 w-40 bg-popover dark:bg-popover text-popover-foreground rounded-xl shadow-md border border-border/30 overflow-hidden z-50">
+                    <div className="grid gap-0 divide-y divide-border/30">
+                        {i18n.locales.map((loc) => (
+                            <button
+                                key={loc}
+                                onClick={() => changeLocale(loc)}
+                                className={`flex items-center gap-2 px-4 py-2 text-sm w-full text-left hover:bg-accent/10 transition-colors ${value === loc ? "bg-accent/10 font-semibold" : ""}`}
+                            >
+                                <span className="flex-1">
+                                    {LABELS[loc] ?? loc}
+                                </span>
+                                {value === loc && (
+                                    <span className="text-xs opacity-70">
+                                        ✓
+                                    </span>
+                                )}
+                            </button>
+                        ))}
+                    </div>
+                </div>
+            )}
+        </div>
+    )
+}
+
+export default function LanguageToggle({
+    className = "",
+}: {
+    className?: string
+}) {
+    return (
+        <Suspense
+            fallback={
+                <button
+                    className="p-2 rounded-full text-muted-foreground opacity-50"
+                    disabled
+                >
+                    <Globe className="w-5 h-5" />
+                </button>
+            }
+        >
+            <LanguageToggleInner className={className} />
+        </Suspense>
+    )
+}