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>

CLAUDE.md

@@ -44,7 +44,7 @@ docs/           — Additional documentation
 
 - **Game** — `name`, `platform` (FK), `status` (u/p/f/r/a), `mastered`, `playtime` (DurationField updated via signal), `year_released`, `sort_name`, `wikidata`
 - **Platform** — `name`, `group`, `icon` (slug, auto-generated from name)
-- **Purchase** — ownership type, prices, currency conversion (`converted_price`, `price_per_game` is a `GeneratedField`), links to Game via M2M. `num_purchases` counts linked games. DLC/SeasonPass/BattlePass must have a `related_game` (the base Game the add-on belongs to; reverse accessor `game.addon_purchases`)
+- **Purchase** — ownership type, prices, currency conversion (`converted_price`, `price_per_game` is a `GeneratedField`), links to Game via M2M. `num_purchases` counts linked games. DLC/SeasonPass/BattlePass must have a `related_game` (the base Game the add-on belongs to; reverse accessor `game.addon_purchases`). **A multi-game Purchase is an *unsplittable* bundle** (one price, whole-purchase refund — e.g. a Humble Bundle). Independently-refundable multi-item orders (e.g. a Steam cart) are modeled as **separate single-game purchases**, not one bundle: the add-purchase form's "separate price per game" mode (≥2 games) creates them, and the row's **Split** action breaks an existing bundle into per-game purchases (price split evenly as a starting point). This is why per-game refund/price need no through-model — each refundable unit is its own Purchase.
 - **Session** — `timestamp_start`/`timestamp_end`, `duration_manual`, `device` (FK), `note`, `emulated`. `duration_calculated` and `duration_total` are `GeneratedField`s (cannot be written directly)
 - **Device** — `name`, `type` (PC/Console/Handheld/Mobile/SBC/Unknown)
 - **PlayEvent** — marks when a game was started/finished (separate from Sessions), `days_to_finish` is a `GeneratedField`