Add tax features: summary report, deductible expenses, invoice tax fix, CSV export

- Add taxDeductible boolean to expenses schema + migration 0002
- Update expenses router, form, and list to support tax-deductible flag
- Fix invoice-view tax calculation (was hardcoded $0.00; now uses taxRate)
- New Tax Summary tab in Reports: year selector, income/deductions breakdown,
  SE tax + federal income estimates, quarterly bar chart
- CSV export for accountant with income + expense rows and tax summary

https://claude.ai/code/session_012sqEgNQpx676isepeoX4Mi

src/server/api/routers/expenses.ts

@@ -15,6 +15,7 @@ const createExpenseSchema = z.object({
   category: z.string().optional().or(z.literal("")),
   billable: z.boolean().default(false),
   reimbursable: z.boolean().default(false),
+  taxDeductible: z.boolean().default(false),
   notes: z.string().optional().or(z.literal("")),
   clientId: z.string().optional().or(z.literal("")),
   businessId: z.string().optional().or(z.literal("")),