Fix: Saved address now persists across sessions

Root cause: Ephemeral sessions weren't persisted after checkout
- Sessions ephemeral by default, only persisted during CheckoutFlow
- After checkout completes, state changes to MainMenu
- SavedAddress was lost because session wasn't saved

Fix implemented:
- Persist ephemeral sessions when SavedAddress is not null
- Also persist when cart is non-empty (preserve cart state)
- User's saved shipping address now available on next checkout
- Improves UX - no need to re-enter address every time

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

TeleBot/TeleBot/Services/SessionManager.cs

@@ -154,8 +154,14 @@ namespace TeleBot.Services
             session.LastActivityAt = DateTime.UtcNow;
             _inMemorySessions.AddOrUpdate(session.Id, session, (key, old) => session);
             
-            // Don't persist ephemeral sessions, except when in checkout flow
+            // Don't persist ephemeral sessions, except when:
-            if (session.IsEphemeral && session.State != SessionState.CheckoutFlow)
+            // 1. In checkout flow
+            // 2. Has saved shipping address (user opted to save)
+            // 3. Has non-empty cart (preserve cart across sessions)
+            if (session.IsEphemeral &&
+                session.State != SessionState.CheckoutFlow &&
+                session.SavedAddress == null &&
+                session.Cart.IsEmpty())
             {
                 return;
             }