Expand electricity cost usage guidance

[rrih]

Summary

• Expand the existing Electricity Cost Calculator page with common monthly appliance examples.
• Add estimation-quality guidance, annualized-cost advice, and FAQ copy for comparison/standby-power queries.
• Keep calculator logic, ad density, AdSense settings, and production scope unchanged.

Selected action

• Type: auto-pr existing-page expansion
• Target: https://rrih.github.io/tools/electricity-cost-calculator/
• Rationale: low-risk launched utility from data/growth/opportunities.json; existing page expansion can add search-supporting copy without regulated formula changes.

Expected KPI impact

• More long-tail appliance electricity cost coverage on an already launched low-risk utility.
• Better topical depth for monthly cost, standby power, appliance comparison, and annualized cost queries.

Verification

• GitHub preflight: success on attempt 1/4; Improve animation generator CTR copy #26 and Add overtime pay calculator #28 were draft / CLEAN / CI success.
• bun test: 27 pass.
• bun --bun tsc --noEmit: pass after clearing stale .next cache.
• bun run lint: pass with existing 9 warnings.
• bun run build: pass, 189 static pages.
• bun run dev-check: pass, dev server reached Ready and was stopped.
• Pre-commit hook: pass.
• Browser: dev server and static export showed the new sections at desktop and mobile widths with no horizontal overflow; static export had 0 console errors for localhost:3334.

Refs #15