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/pages/Salarios.tsx

@@ -64,7 +64,7 @@ export default function Salarios() {
         accessorKey: 'amount',
         header: ({ column }) => <DataTableColumnHeader column={column} title="Monto" />,
         cell: ({ row }) => (
-          <span className="font-mono font-bold text-primary">
+          <span data-sensitive className="font-mono font-bold text-primary">
             +{formatAmount(row.original.amount, row.original.currency)}
           </span>
         ),
@@ -126,7 +126,7 @@ export default function Salarios() {
                 <Banknote className="w-4 h-4" />
                 <span className="text-xs font-medium uppercase tracking-wider">Total acumulado</span>
               </div>
-              <span className="text-2xl font-bold font-mono text-primary">
+              <span data-sensitive className="text-2xl font-bold font-mono text-primary">
                 {formatAmount(summary.total_amount, 'CRC')}
               </span>
             </CardContent>