fix(accounting): tax report degrades gracefully if cost side fails (+ surface the error)

The cost side is supplementary — it must never 500 the core revenue report.
getTaxReport now wraps loadCosts in try/catch: on failure it returns empty
costs + a costsError string and logs the real error. The tax page shows the
revenue report plus a non-fatal amber banner with the cost-side error message,
so the actual cause is visible in the UI instead of an opaque 500.

frontend/src/services/taxReport.service.ts

@@ -97,6 +97,8 @@ export interface TaxReport {
   /** Cost side (#4). Present when the accounting tables exist; empty
    *  otherwise. */
   costs: TaxReportCosts;
+  /** Non-fatal: set when the cost side failed to load (revenue still shown). */
+  costsError?: string | null;
   /** Income/cost/result summary (#4). */
   summary: TaxReportSummary;
   currency: string;