fix: 批改页 J/K 按键位对调；预览换内容不再进浏览器历史，“后退”能回到作品墙

- J 上一个、K 下一个，和面板上的左右箭头一致
- 打开的作品放进地址栏（?sub=）：点开记一步历史，J/K 切换只替换，关闭 / Esc 退回那一步
- 所有会换内容的预览 iframe 加 key，换内容时换新的 iframe：在原 iframe 里改 srcdoc 是一次导航，
  会进浏览器历史，“后退”就成了在预览里倒退（批改页切换学生、挑战页改代码都会这样）

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -32,7 +32,7 @@ The backend was rewritten from Django in 2026-10 to cut memory use (~25 MB vs. 1
   - `src/icons.ts` — `<Icon icon="lucide:name" />` renders bundled `lucide-vue-next` icons (works offline); register new names in its map. Fonts are bundled too (`@fontsource/jetbrains-mono`)
   - `components/workbench/Workbench.vue` — shared challenge / creation page: left = brief + AI chat (`PromptPanel`), right = preview / code (`CodeTabs`) + `VersionBar`. Every assistant message with code is a version; hand edits and pasted external-AI code are saved as versions via `POST /prompt/versions` (`source` `edit` / `manual`, stored as a user+assistant pair). Challenges are submitted only when the student clicks “交这一版” (`POST /submission/` with `message_id`), never automatically. Editor code is per task (`composables/useTaskCode.ts`, localStorage `web-code:<user>:<task>`); the tutorial page edits the shared `store/editors.ts` refs but `Workspace.vue` saves them under the same per-task key and `TutorialContent` restores them (example code only the first time)
   - `pages/ChallengeHome.vue` — challenge list page; `pages/TaskEditor.vue` (prop `kind`) is the single tutorial / challenge authoring page
-  - `pages/Review.vue` (“批改”, default teaching page) + `components/review/ReviewPanel.vue` (also opened from the gradebook) — opens as a large preview (wall hidden, Esc returns; “小面板” switches to a side panel, remembered); keys: J/K next/prev student, 1–5 rate, P present; “全部班级” groups the wall by class; `components/review/PeersDrawer.vue` is the students' “同学作品” (本班 / 全部班级)
+  - `pages/Review.vue` (“批改”, default teaching page) + `components/review/ReviewPanel.vue` (also opened from the gradebook) — opens as a large preview (wall hidden, Esc returns; “小面板” switches to a side panel, remembered); keys: J/K previous/next student (by key position), 1–5 rate, P present; “全部班级” groups the wall by class; `components/review/PeersDrawer.vue` is the students' “同学作品” (本班 / 全部班级)
   - `components/layout/AppShell.vue` — top bar for every page; `pages/Dashboard.vue` is the teaching sidebar (“教学”) under `/dashboard`
   - `src/store/` — Reactive state (plain Vue refs, not Vuex/Pinia); `prompt.ts` / `guidance.ts` own the WebSockets
   - `src/pages/`, `src/components/` — user management uses `components/dashboard/UserFormModal.vue` (there is no Django Admin anymore)