[POC V1] T6.2 Add manual refresh interaction

[fcarucci]

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

Phase: P6 Error Handling And Polish

Source task: T6.2

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Purpose:

• keep the POC aligned with server-functions-first architecture

Output:

• refresh button that re-runs get_agent_graph_snapshot()

Tests:

• component test: clicking refresh triggers a refetch
• integration test: refreshed data replaces the old snapshot
• integration test: refresh failure preserves the last good graph while showing an error state

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