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/templates/icons/split.html

@@ -0,0 +1,5 @@
+<svg xmlns="http://www.w3.org/2000/svg"
+     viewBox="0 0 24 24"
+     class="text-black dark:text-white w-4 h-4">
+    <path fill="currentColor" d="M14 4l2.29 2.29-2.88 2.88 1.42 1.42 2.88-2.88L20 12V4z M10 4H4v8l2.29-2.29 4.71 4.71V20h2v-8.41l-5.29-5.3z"></path>
+</svg>