fix: resolve login persistence bug and aggressive logout on network blips

- Updated Login.tsx to automatically redirect users to the dashboard if a valid token is already present in localStorage.

- Refactored getUserById to properly throw server/network errors instead of silently returning undefined.

- Updated AuthGuard in App.tsx to gracefully handle network errors without destroying the user's valid localStorage tokens.

services/dataService.ts

@@ -280,16 +280,20 @@ export const getUserById = async (id: string): Promise<User | undefined> => {
     const response = await fetch(`${API_URL}/users/${id}`, {
       headers: getHeaders()
     });
-    if (!response.ok) return undefined;
-    
+    if (!response.ok) {
+       if (response.status === 401 || response.status === 403 || response.status === 404) {
+         return undefined; // Invalid user or token
+       }
+       throw new Error(`Server error: ${response.status}`);
+    }
+
     const contentType = response.headers.get("content-type");
     if (contentType && contentType.indexOf("application/json") !== -1) {
       return await response.json();
     }
-    return undefined;
   } catch (error) {
     console.error("API Error (getUserById):", error);
-    return undefined;
+    throw error; // Rethrow so AuthGuard catches it and doesn't wipe tokens
   }
 };