ui: Migrate Explorer core entity tabs to API

[victorquinn]

Summary

Migrate the Explorer's ISO, RTO, Balancing Authority, and Region tabs from static imports to API-backed data. This is the beginning of the high-risk Explorer migration.

Components to Update

• Explorer ISO tab → fetch from /api/v1/isos
• Explorer RTO tab → fetch from /api/v1/rtos
• Explorer BA tab → fetch from /api/v1/balancing-authorities
• Explorer Regions tab → fetch from /api/v1/regions

Implementation

• Server Components for initial data load
• Client-side filtering for instant feedback (small datasets)
• Feature flag gated
• ExplorerContext updated to support API data source

Acceptance Criteria

• All 4 entity tabs functional with API data
• Client-side filtering still instant
• Map interactions work correctly
• Feature flag rollback to JSON works
• npm run build passes

Spec ref: Section 9, Phase 2b
Blocked by: #15 (core entity endpoints), #4 (feature flags)
Blocks: Utility Explorer migration

[meridian-texture]

Triage note (2026-09-18): keeping this open — partially done, and the remaining part is real.

Done: utilities, power plants, pricing nodes, transmission, substations, EV charging and programs panels all read from /api/v1, and the read endpoints /api/v1/isos, /api/v1/rtos, /api/v1/balancing-authorities and /api/v1/regions all exist.

Not done: the grid-operator tab and the map still read those same entities from bundled JSON via @/lib/data:

• components/explorer/panels/GridOperatorListPanel.tsx — getAllBalancingAuthorities, getAllIsos, getAllRtos
• components/explorer/ExplorerPanel.tsx — same three
• components/explorer/ExplorerMap.tsx — plus getAllPrograms, getRegionById
• components/explorer/panels/{ProgramDetailPanel,UtilityDetailPanel}.tsx — getRegionById

The endpoints these need are already live, so this is a swap to useSWR against them, not new API work. ISOs/RTOs/BAs are small, slow-changing sets, so the payload argument is weaker here than it was for utilities — but they are a second source of truth that drifts from Postgres, which is the actual reason to finish it.