Adds authentication and pricing pages for user onboarding

Introduces comprehensive user authentication flow with plan selection and registration

Includes:
- Pricing page with plan details and selection
- Payment page with plan summary and card information
- Signup page for different plan tiers
- Shared authentication layout and design system improvements

Enhances user onboarding experience with clear plan information and streamlined signup process

PlanTempus.Application/Features/Accounts/Pages/Payment.cshtml.cs

@@ -0,0 +1,14 @@
+using Microsoft.AspNetCore.Mvc.RazorPages;
+using PlanTempus.Application.Features.Accounts.Models;
+
+namespace PlanTempus.Application.Features.Accounts.Pages;
+
+public class PaymentModel : PageModel
+{
+    public PlanInfo SelectedPlan { get; private set; } = PlanCatalog.Pro;
+
+    public void OnGet(string plan = "pro")
+    {
+        SelectedPlan = PlanCatalog.GetPlan(plan);
+    }
+}