fix: coerce numeric/decimal columns in repository mappers (refs #241)

node-postgres returns numeric/decimal columns as JavaScript strings,
but the TypeScript interfaces for MaintenanceRecord and OwnershipCost
declare numeric fields as number. The mappers were passing values
through raw, breaking type-safe arithmetic and display (e.g. the
amount column on the vehicle summary screen was empty until the
recent frontend workaround in PR #240, and OwnershipCostsList silently
no-ops toLocaleString on the string).

Backend
- mapMaintenanceRecord: coerce cost via Number() when non-null.
- ownership-costs mapRow: coerce amount via Number().

Frontend (remove now-redundant workarounds)
- MaintenanceRecordsList: drop Number() coercion on cost and
  odometerReading; use the number values directly.
- VehicleDetailPage / VehicleDetailMobile: revert the PR #240 cost
  coercion to the simple typeof number guard now that the backend
  honors the type.

Scope notes
- Other repositories with the same pattern (stations, community-stations,
  fuel-logs enhanced methods) are tracked separately because they have
  unclear downstream consumers and warrant their own investigation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/src/features/ownership-costs/data/ownership-costs.repository.ts

@@ -16,7 +16,8 @@ export class OwnershipCostsRepository {
       vehicleId: row.vehicle_id,
       documentId: row.document_id,
       costType: row.cost_type,
-      amount: row.amount,
+      // node-postgres returns numeric/decimal columns as strings; coerce to honor the number type.
+      amount: Number(row.amount),
       description: row.description,
       periodStart: row.period_start,
       periodEnd: row.period_end,