Add SmartBizCalc to Accounting/Invoicing section

[galmorduku]

SmartBizCalc (https://www.smartbizcalc.com) is a free suite of 410+ business calculators covering:

• Tax: self-employment tax, payroll tax, S-corp salary, quarterly estimated tax
• Insurance costs: general liability, workers comp, commercial auto, professional liability
• Contractor and service pricing: HVAC, plumbing, electrician, cleaning, lawn care
• Business financials: break-even, food truck profit, startup cost, business valuation

No signup required. All calculators run client-side. Fits naturally in the Accounting/Invoicing section as a free calculation resource for small business owners and freelancers.