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/pages/admin/clients/TaxReportPage.tsx

@@ -358,6 +358,19 @@ export const TaxReportPage: React.FC = () => {
         )}
       </div>
 
+      {/* Non-fatal: the revenue report loaded but the cost side errored. */}
+      {report?.costsError && (
+        <Card padding="md">
+          <div className="flex items-start gap-3 text-amber-700 dark:text-amber-400">
+            <AlertCircle className="w-5 h-5 flex-shrink-0 mt-0.5" />
+            <div>
+              <p className="font-medium">{t('taxReport.costsErrorTitle', 'Costs could not be loaded')}</p>
+              <p className="text-sm text-neutral-600 dark:text-neutral-400 mt-1 break-words">{report.costsError}</p>
+            </div>
+          </div>
+        </Card>
+      )}
+
       {/* Results */}
       {isLoading ? (
         <Card padding="lg"><Loading /></Card>

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;