Add Agents UI page + installer script endpoint

- client/src/Agents.js: page showing registered Linux agents with live
  status, CPU/memory/disk gauge bars, load/uptime/network stats, and
  an install-instructions panel with copyable curl commands
- client/src/Agents.css: dark-theme card grid styles matching the app
- client/src/App.js: adds /agents route and Agents nav link
- server/index.js: adds GET /api/agents/installer to serve install-agent.sh
- client/build: production build updated

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

client/build/static/css/main.e143c118.css.map

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