Add privacy mode toggle to blur sensitive financial amounts

Eye/EyeOff icon next to theme toggle. Persists in localStorage.
Applies CSS blur to all elements marked with data-sensitive attribute
across Dashboard, Budget, Pensions, Salarios, and Transactions pages.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/DashboardSection.tsx

@@ -58,7 +58,7 @@ export default function DashboardSection({
                 {settings.label}
               </span>
               {total != null && totalCurrency && (
-                <span className="text-sm font-bold font-mono text-foreground">
+                <span data-sensitive className="text-sm font-bold font-mono text-foreground">
                   {formatAmount(total, totalCurrency)}
                 </span>
               )}