feat(integrations,classisland,server,web,desktop): ClassIsland 联动插件——上报课表与上课状态、下发提醒并回执、教室端推送通道

教室机器上的 ClassIsland 现在可以直接和班级小助手互通：插件走 /api/integrations/classisland/*，用设备令牌（X-ClassIsland-Token，库里只存 sha256）独立鉴权，不占用 JWT 通道。

- 服务端：新增 integrations 模块（上报 / 待弹出 / 回执 / 课表镜像 / 回写），两个迁移（设备令牌表、Class.notificationChannel）；通知、叫人、联动页下发统一复用 pushToClassIsland，避免"某个页面发的通知不联动"
- 插件：packages/classisland-plugin（.NET 8，独立于 pnpm workspace）——BridgeService 上报与提醒轮询、ScheduleMapper/ClassPlanWriter 课表镜像、Avalonia 设置页、ClassHelperNotificationProvider 在 ClassIsland 上弹出提醒；本地/内网地址强制直连，躲开系统代理返回的假 404
- 客户端：设置页可选提醒通道（both / client / classisland），并订阅相应实时事件
- Web 端：新增「ClassIsland 联动」页（设备管理与提醒下发）
- 校验与打包：verify:classisland（需后端）、verify:classisland-plugin（静态契约 54 项）、build/dist:classisland-plugin 产出 .cipx
- 文档：README 与 AGENTS.md 同步新增联动说明，docs/screenshots/classisland 留三张真机截图

门禁：pnpm lint 0 error、pnpm typecheck 四包通过、verify:classisland-plugin 54/54 通过。运行期仍需在装了 ClassIsland 的机器上实测（步骤见 AGENTS.md §8）。

package.json

@@ -15,6 +15,7 @@
     "dev:desktop": "pnpm build:shared && pnpm --filter @classhelper/desktop-client dev",
     "build": "pnpm build:shared && pnpm --filter @classhelper/server build && pnpm --filter @classhelper/web-admin build && pnpm --filter @classhelper/desktop-client build",
     "build:desktop": "pnpm build:shared && pnpm --filter @classhelper/desktop-client build",
+    "build:classisland-plugin": "node packages/classisland-plugin/scripts/build.mjs",
     "typecheck": "pnpm build:shared && pnpm -r --no-bail typecheck",
     "lint": "eslint .",
     "lint:fix": "eslint . --fix",
@@ -31,11 +32,14 @@
     "verify:desktop": "pnpm --filter @classhelper/desktop-client smoke",
     "verify:web": "node scripts/ui-smoke/run.mjs",
     "verify:packaged": "node scripts/verify-packaged.mjs",
+    "verify:classisland": "pnpm --filter @classhelper/server verify:classisland",
+    "verify:classisland-plugin": "node packages/classisland-plugin/scripts/verify.mjs",
     "icons": "node scripts/generate-icons.mjs",
     "dist:win": "pnpm --filter @classhelper/desktop-client dist:win",
     "dist:dir": "pnpm --filter @classhelper/desktop-client dist:dir",
     "dist:server": "pnpm build:shared && pnpm --filter @classhelper/server build && pnpm --filter @classhelper/web-admin build && node scripts/dist-server.mjs",
-    "dist:all": "pnpm dist:server && pnpm dist:win"
+    "dist:classisland-plugin": "node packages/classisland-plugin/scripts/build.mjs --cipx",
+    "dist:all": "pnpm dist:server && pnpm dist:win && pnpm dist:classisland-plugin"
   },
   "devDependencies": {
     "concurrently": "^10.0.5",