Fix TypeError: Decimal + float in cart_total computation

ticket_total() returned float while total() returns Decimal, causing
a crash when summing cart totals. Standardise all total functions to
return Decimal.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 import path_setup  # noqa: F401  # adds shared_lib to sys.path
 
+from decimal import Decimal
 from pathlib import Path
 
 from quart import g, abort
@@ -61,7 +62,7 @@ async def cart_context() -> dict:
     # Global counts for cart-mini (always global)
     cart_qty = sum(ci.quantity for ci in all_cart) if all_cart else 0
     ctx["cart_count"] = cart_qty + len(all_cal) + len(all_tickets)
-    ctx["cart_total"] = (total(all_cart) or 0) + (calendar_total(all_cal) or 0) + (ticket_total(all_tickets) or 0)
+    ctx["cart_total"] = (total(all_cart) or Decimal(0)) + (calendar_total(all_cal) or Decimal(0)) + (ticket_total(all_tickets) or Decimal(0))
 
     # Page-scoped data when viewing a page cart
     page_post = getattr(g, "page_post", None)