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/urls.py

@@ -105,6 +105,16 @@ urlpatterns = [
         purchase.refund_purchase,
         name="refund_purchase",
     ),
+    path(
+        "purchase/<int:purchase_id>/split/confirm",
+        purchase.split_purchase_confirmation,
+        name="split_purchase_confirmation",
+    ),
+    path(
+        "purchase/<int:purchase_id>/split",
+        purchase.split_purchase,
+        name="split_purchase",
+    ),
     path("session/add", session.add_session, name="add_session"),
     path(
         "session/add/for-game/<int:game_id>",