Model refundable orders as separate purchases; add split action

A multi-game Purchase is now treated as an *unsplittable* bundle (one
price, whole-purchase refund). Independently-refundable multi-item orders
(e.g. a Steam cart) are instead recorded as N separate single-game
purchases, so per-game pricing and per-game refunds work with the
existing single-purchase machinery — no through-model needed.

Add-purchase form (single form, single endpoint):
- 1 game: unchanged.
- 2+ games: a "Separate price per game" toggle appears (default off =
  one bundle price). On, the bundle Price hides and one price input per
  game appears; the view creates one single-game Purchase each from
  price_for_game_<id>. `price` is now optional so combined mode still
  validates.

Split action:
- A Split button on multi-game purchase rows opens a confirmation modal
  that replaces the bundle with one single-game purchase per game (price
  split evenly, needs_price_update set), then HX-Redirects to the list.

New general-purpose `selection-fields` custom element renders one synced
form field per selected item of a source SearchSelect (consuming the
existing search-select:change contract); it knows nothing about prices,
so it is reusable. Behavior in ts/elements/selection-fields.ts.

Adds the bundle-vs-separate-purchases convention to CLAUDE.md, a split
icon, and unit + Playwright e2e coverage.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

games/forms.py

@@ -231,6 +231,9 @@ class PurchaseForm(PrimitiveWidgetsMixin, forms.ModelForm):
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
         self.fields["platform"].queryset = Platform.objects.order_by("name")
+        # The bundle Price is optional: in price-per-game mode it is hidden and
+        # the per-game inputs carry the prices instead. Empty falls back to 0.
+        self.fields["price"].required = False
 
     games = MultipleGameChoiceField(
         queryset=Game.objects.order_by("sort_name"),
@@ -305,6 +308,11 @@ class PurchaseForm(PrimitiveWidgetsMixin, forms.ModelForm):
                 )
             if not name:
                 self.add_error("name", f"{type_display} must have a name.")
+
+        # An empty bundle Price (price-per-game mode) saves as 0, not NULL.
+        if cleaned_data.get("price") is None:
+            cleaned_data["price"] = 0
+
         return cleaned_data