fix(devices): clarify offline alert and heartbeat stale states

[mislayming]

Summary

• Add a derived device presence helper that prioritizes unresolved device_offline alerts over tunnel status.
• Show separate Offline alert and Heartbeat stale states on the Devices page so operators do not see a misleading plain Online state while Alerts reports the device offline.
• Add focused unit coverage for alert priority and heartbeat staleness.

Validation

• cd web && npm run test -- deviceStatus.test.ts
• cd web && npm run typecheck
• cd web && npm run build
• cd web && npx eslint src/lib/deviceStatus.ts src/lib/deviceStatus.test.ts src/pages/Edges.tsx --ext ts,tsx --report-unused-disable-directives passes with 0 errors and 1 existing warning in Edges.tsx.
• cd web && npm run lint still fails on pre-existing unrelated repo issues such as src/api/chat.ts, src/components/ChatInput.tsx, src/components/MessageBubble.tsx, src/pages/DeviceShell.tsx, and src/pages/SkillRun.tsx.

Author confirmation

• I have read and agree to the project contribution guide in CONTRIBUTING.md.

[singchia]

这个方向可以：把 Device offline alert、心跳超时、tunnel 在线状态区分开，确实能减少设备页和告警页互相打架的感觉。

但这个 PR 需要先 rebase / 重做一下。我们最近已经调整过设备列表页面，现在列表是 Device-first：页面主数据来自 /devices，Edge 只是挂在 Device 下面的 hostEdge/agent 连接信息。#166 现在还是按旧的 Edge row 模型改 web/src/pages/Edges.tsx，所以会和 main 上的设备列表冲突，也容易把 Device/Edge 语义又混回去。

建议改法：

• 状态列基于 Device 行来判断：device.online / device.last_seen_at 是设备状态基准；
• hostEdge.status 只表示 Edge tunnel/agent 连接状态，不要再当成设备在线状态；
• device_offline 未恢复告警可以覆盖显示成 Offline alert，但要覆盖 open / acknowledged / silenced，不能只看 open；
• 不要只拉前 100 条 incident，否则告警多时设备状态会漏。

另外这是 UI 行为调整，后续类似 PR 麻烦带上涉及页面的截图，至少包括设备列表状态列的 before/after。这个改动用户一眼就会看到，截图能让 review 更快判断文案、颜色、状态优先级是不是对。

[singchia]

Reviewed. I would not merge this as-is.

The branch is now dirty against main and edits the old Devices/Edges page surface. Since main has moved further toward a Device-first list and Edge-operation wording, this will conflict with the current UI model. Please rebase and update the implementation against the current page, then include screenshots for the affected Devices states: normal online, stale heartbeat, and offline alert.

The status helper idea is still useful; it just needs to land on the current UI baseline.