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/transactions/transaction-columns.tsx

@@ -88,6 +88,7 @@ export function getTransactionColumns({
         const tx = row.original;
         return (
           <span
+            data-sensitive
             className={cn(
               'font-mono font-medium',
               tx.transaction_type !== 'COMPRA' && 'text-primary',