[POC V1] T6.1 Add connection-state UX

[fcarucci]

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

Phase: P6 Error Handling And Polish

Source task: T6.1

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Purpose:

• make failures legible during demos and local development

Output:

• clear loading, empty, degraded, and disconnected states

Tests:

• component test: disconnected gateway renders a recovery message
• component test: malformed snapshot does not crash the page
• component test: partial data still renders available nodes and summaries

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[fcarucci]

Linked pull request: #57