feat(frontend/dashboard): Potential Monthly Savings card mirrors Recommendations-page range

[cristim]

Problem

PR #283 closed #279 by making every summary card on the
Recommendations page render a per-cell range (savings, upfront,
payback) and narrow on selection. The Dashboard view's "Potential
Monthly Savings" card (frontend/src/dashboard.ts:128) still renders
the flat summary.potential_monthly_savings from
/api/dashboard/summary — the same overcounting bug class #272 closed
on the Recommendations page (sums every variant of every cell, ~6×
overstates achievable totals).

Splitting this out of #279 because the Dashboard work is structurally
different from the Recommendations-page work that's already shipped:
it's a separate page, has no per-row selection state to narrow on, and
needs either a backend /dashboard/summary extension or a frontend
extra-fetch to source the per-cell range data.

Acceptance criteria

• Dashboard's "Potential Monthly Savings" card renders the same
  per-cell range that the Recommendations page card does
  (pageLevelRange(groupRecsByCell(recs)).savingsMin /
  .savingsMax).
• Implementation: frontend-only — dashboard.ts::loadDashboard
  fetches /api/recommendations alongside the existing
  /api/dashboard/summary + /api/upcoming calls, computes the range
  client-side using the existing pageLevelRange /
  groupRecsByCell helpers exported from recommendations.ts. The
  recs endpoint is served from Postgres cache so the extra API call
  is cheap. Backend change deferred — a future PR can move the range
  computation server-side if needed.
• No selection narrowing on Dashboard — Dashboard is a read-only
  view, no checkboxes. Card always reflects the range across whatever
  recommendations the user's current provider/account filter exposes.
• Other Dashboard cards (Active Commitments, Current Coverage,
  YTD Savings) keep their existing API-driven values — they're
  aggregates that don't have the variant-fan-out overcounting issue.
• If /api/recommendations fails, the Dashboard still renders —
  the savings card falls back to formatCurrency(0) and the rest of
  the dashboard is unaffected (independent loaders).
• Tests:
  • dashboard.test.ts (or wherever dashboard tests live): assert
    the savings card renders the range when recs are mocked.
  • Failure-isolation: when getRecommendations rejects, the other
    cards still render.

References

• ux(frontend/recs): Potential Monthly Savings card sums all variants — should mirror Recommended range #272 — closed the same bug class on the Recommendations page.
• feat(frontend): summary cards + 'Showing X of Y' narrow on selection in real time #279 — broadened the fix to all 4 cards on Recommendations (PR feat(frontend/recs): summary cards narrow on selection + drop redundant banner (closes #278 #279) #283).
• pageLevelRange / groupRecsByCell in
  frontend/src/recommendations.ts (exported).