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/TransactionList.tsx

@@ -128,6 +128,7 @@ export default function TransactionList({
                   </p>
                 </div>
                 <span
+                  data-sensitive
                   className={cn(
                     'font-mono text-sm font-medium shrink-0',
                     tx.transaction_type === 'DEVOLUCION' && 'text-primary'