ui: Migrate pricing-nodes + programs + transmission-lines pages to API

[victorquinn]

Summary

Migrate the three standalone (non-Explorer) pages from static JSON imports to API-backed data fetching. These are the lowest-risk migrations.

Pages to Migrate

1. app/(shell)/pricing-nodes/page.tsx — List page
2. app/(shell)/pricing-nodes/[slug]/page.tsx — Detail page
3. app/(shell)/power-plants/page.tsx — Already uses client fetch, migrate to SWR + API (SKIP — separate issue)
4. Programs page (if it exists in the app directory)
5. app/(shell)/transmission-lines/page.tsx — List page

Implementation Pattern

• Check feature flag: if 'json', use existing static import; if 'database', use API
• Server Components fetch from API with cache tags
• Client Components use SWR for interactive filtering
• Skeleton loading states during data fetch
• On-demand revalidation via cache tags (NOT time-based ISR)

Acceptance Criteria

• All three pages work with feature flag set to 'database'
• All three pages still work with feature flag set to 'json' (rollback)
• No visual regressions
• Skeleton loading states during fetch
• LCP <2.5s
• npm run build passes

Spec ref: Sections 6.1–6.4
Blocked by: #4, #10, #11, #12 (feature flags, API endpoints), #18 (skeletons)
Blocks: Nothing