fix: highlight mixed agent status in dashboard

[newbish]

Summary

• show a yellow mixed-state status badge when a multi-agent selection includes one or more agents that are no longer actively sending data
• keep the badge green only when every selected agent is healthy, and keep red when every selected agent is stale
• replace the native details dropdown so the agent selector closes when you click outside it or press Escape
• add dashboard-agent-status coverage for the new aggregate status logic

Test Plan

• cd apps/dashboard && npm test -- --run src/lib/dashboard-agent-status.test.ts
• cd apps/dashboard && npm run typecheck
• cd apps/dashboard && npm run build

[github-actions]

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