fix: coerce maintenance cost to number for amount column (refs #239)

Postgres numeric columns come back as strings via node-postgres, so
typeof rec.cost === 'number' was false and the amount column rendered
as '—'. Coerce with Number() (matching the pattern in
MaintenanceRecordsList) so the cost displays as a dollar amount.

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

frontend/src/features/vehicles/mobile/VehicleDetailMobile.tsx

@@ -142,7 +142,9 @@ export const VehicleDetailMobile: React.FC<VehicleDetailMobileProps> = ({
         secondaryParts.push(new Date(rec.date).toLocaleDateString());
         secondaryParts.push('Maintenance');
         const secondary = secondaryParts.join(' • ');
-        const amount = typeof rec.cost === 'number' ? `$${rec.cost.toFixed(2)}` : undefined;
+        // Backend returns numeric/decimal columns as strings via node-postgres; coerce.
+        const costNum = rec.cost != null ? Number(rec.cost) : NaN;
+        const amount = Number.isFinite(costNum) ? `$${costNum.toFixed(2)}` : undefined;
         list.push({
           id: rec.id,
           type: 'Maintenance',