[POC V1] T5.5 Build gateway and graph summary cards

[fcarucci]

Imported from docs/milestones/proof-of-concept-1/poc_v1_task_breakdown.md.

Phase: P5 Vertical UI Slice

Source task: T5.5

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Purpose:

• make the dashboard useful even before graph inspection

Output:

• summary cards for gateway status, agent count, active agent count, and edge count

Tests:

• component test: summary values match the snapshot fixture
• component test: degraded gateway state is reflected in the status summary card

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[fcarucci]

Linked pull request: #56