[T5.5] Build gateway and graph summary cards

[fcarucci]

Task: #28 [POC V1] T5.5 Build gateway and graph summary cards

Closes #28

Summary

• add a summary-card row to the dashboard for gateway health, agent count, active agents, and connections
• derive graph totals from the assembled snapshot so the cards stay aligned with the rendered graph
• cover degraded gateway and summary-value behavior with focused dashboard tests

Verification

• cargo fmt --all
• npm run build:css
• cargo check --features server
• cargo test
• npm run verify:live -- --url http://127.0.0.1:4127/ --screenshot /tmp/t55-dashboard.png --dom /tmp/t55-dashboard.html --video t55-dashboard.mp4 --wait-text "Gateway status" --wait-text "Connections" --wait-connected

[fcarucci]

Live / verification passed.

• Artifact: https://github.com/fcarucci/daneel/releases/download/untagged-494cf68a20034af2dc7c/home_20260319_093606.mp4
• Latest session cards detected: 0
• Connected ribbon present: true
• Screenshot: /tmp/t55-dashboard.png
• DOM snapshot: /tmp/t55-dashboard.html