feat(clients): implement client management actions and UI

- Added actions for creating, updating, and archiving client records in `actions.ts`.
- Created a new client management UI in `clients-client.tsx` to display and manage clients.
- Integrated client data fetching and state management in `page.tsx`.
- Updated layout to change fallback user name from "MindSpace Kullanıcısı" to "Cognis Kullanıcısı".

app/(dashboard)/layout.tsx

@@ -19,7 +19,7 @@ export default async function DashboardLayout({
         .maybeSingle()
     : { data: null };
 
-  const fallbackName = user?.email?.split("@")[0] ?? "MindSpace Kullanıcısı";
+  const fallbackName = user?.email?.split("@")[0] ?? "Cognis Kullanıcısı";
   const displayName =
     [profile?.first_name, profile?.last_name].filter(Boolean).join(" ") ||
     fallbackName;