Add panel visualization

[R055LE]

Summary

• replace the plain circuit table with an ordered breaker directory
• show pole count, mapping coverage, verification state, and parent/subpanel relationships
• open mapped breakers on the correct floorplan with the circuit selected
• keep unmapped navigation disabled and make the directory usable at phone widths
• add deterministic browser coverage for panel behavior and mobile containment

Why

Hearth already stored panels, circuits, circuit points, pole counts, and feed relationships, but the panel screen did not make that inventory useful. The schema does not store physical row or left/right breaker positions, so this presents an honest one-column directory instead of inventing a physical panel layout.

Security posture

This reuses the existing circuit-point list endpoint and React text rendering. It adds no migration, upload surface, runtime dependency, external service, or authentication claim. Hearth's private-network boundary remains load-bearing.

Validation

• frontend unit tests
• frontend lint and production build
• 9 Playwright Chromium tests
• 23 backend tests with 83% coverage
• production-image agent-browser checks at desktop and 390 px phone widths
• clean browser console and page-error checks
• hardened runtime check with non-root user, read-only root filesystem, dropped capabilities, and no-new-privileges
• Trivy HIGH/CRITICAL scan: 0 findings

Closes #36