Agent: resolve "today" in the browser's timezone, not hardcoded CR

The CR-hardcoded today broke two scenarios: the owner traveling, and
any future non-CR user. Now the provider sends the browser's IANA
timezone (X-Client-Timezone) with every CopilotKit request — the
runtime forwards x-* headers to the agent on its own — and the backend
binds it to a per-request ContextVar next to the DB session.
get_current_date and the future-cuota bounds use it; the tool also
reports the timezone so the model can echo it. Unknown or absent zones
(n8n posts, tests) fall back to Costa Rica; comma-joined duplicate
header values are tolerated.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -77,7 +77,17 @@ export default function App() {
         <PrivacyProvider>
           <AuthProvider>
             <QueryClientProvider client={queryClient}>
-              <CopilotKit runtimeUrl="/api/copilotkit" agent="wealthysmart">
+              <CopilotKit
+                runtimeUrl="/api/copilotkit"
+                agent="wealthysmart"
+                headers={{
+                  // Lets the agent resolve "hoy"/"este mes" in the user's
+                  // local time wherever they are; backend falls back to
+                  // Costa Rica when absent.
+                  "X-Client-Timezone":
+                    Intl.DateTimeFormat().resolvedOptions().timeZone,
+                }}
+              >
                 <AppRoutes />
                 <Toaster richColors position="top-right" closeButton />
               </CopilotKit>