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/index.css

@@ -129,3 +129,10 @@
     @apply font-sans;
     }
 }
+
+/* Privacy mode: blur sensitive financial data */
+.privacy [data-sensitive] {
+  filter: blur(8px);
+  user-select: none;
+  transition: filter 0.2s ease;
+}