refactor(exams): redesign exam creation page with 3-mode selector

- Replace cramped 3-column grid with vertical layout
- Add 3 large selectable cards at top: Manual / AI / Rich Text Editor
- Rich Text Editor mode redirects to /teacher/exams/new
- Basic info form is now always visible (not hidden in AI mode)
- Exam mode config always visible at bottom
- Add "rich" to mode enum with validation bypass
- Replace all hardcoded English/Chinese strings with i18n keys
- Add 20+ new i18n keys to zh-CN and en (mode labels, descriptions, actions)
- Clean up mixed-language UI text

src/app/(dashboard)/teacher/exams/create/page.tsx

@@ -1,18 +1,18 @@
 import type { JSX } from "react"
+import { getTranslations } from "next-intl/server"
 import { ExamForm } from "@/modules/exams/components/exam-form"
 
 export const dynamic = "force-dynamic"
 
-export default function CreateExamPage(): JSX.Element {
+export default async function CreateExamPage(): Promise<JSX.Element> {
+  const t = await getTranslations("examHomework")
   return (
-    <div className="flex w-full justify-center items-center min-h-[calc(100vh-160px)] p-8 max-w-[1200px] mx-auto">
-      <div className="w-full space-y-6">
-        <div>
-          <h1 className="text-2xl font-bold tracking-tight">Create Exam</h1>
-          <p className="text-muted-foreground">Configure a new exam for your classes.</p>
-        </div>
-        <ExamForm />
+    <div className="mx-auto w-full max-w-[1200px] space-y-6 p-6">
+      <div>
+        <h1 className="text-2xl font-bold tracking-tight">{t("exam.form.createTitle")}</h1>
+        <p className="text-muted-foreground">{t("exam.form.createDescription")}</p>
       </div>
+      <ExamForm />
     </div>
   )
 }