fix(server): own game-account signup, and repair the gate slice 1 broke

`POST /player/shard/account` and its staff twin have answered 500 for every
caller since slice 1: the ported controller called
`settings.isGameAccountSignupEnabled()`, which is a member of core's settings
model and not of `ctx.settings` — three functions, deliberately. The call was
`undefined(...)`, the TypeError landed in the catch, and no test reached the
branch.

The gate now lives on the side that uses it (`utils/gameSignup.js`), which is
also where the policy belongs: the setting's own help text names Bridge.cfg and
says the shard's SignupMode must agree, and core cannot own a sentence about a
UO shard. The admin field moves to this module's Shard page and the derived
flag onto `/public/shard/features`, beside the visibility flags the same
callers already read.

The setting KEY is unchanged. Renaming `game_account_signup` would silently
reset every configured instance to `disabled` on upgrade, with players
reporting broken signup as the only clue — the same grandfathering as
`spawn_atlas_servuo_path` and the seven stream ids.

Both regression tests were shown to fail against the bug before it was fixed.

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

server/utils/gameSignup.js

@@ -0,0 +1,63 @@
+// ── Whether this site creates game accounts, and in which direction ────────
+//
+// This policy was core's until slice 3 of the Phase 3 extraction, and it should
+// never have been: the setting's own help text names *Bridge.cfg* and says the
+// game server's `SignupMode` must agree with it. That is a sentence about a UO
+// shard, and core cannot own a sentence about a UO shard.
+//
+// **The setting key is unchanged.** `game_account_signup` keeps its name and its
+// row in core's `settings` table, read and written through `ctx.settings`. The
+// key is not prefixed because renaming it would silently reset every existing
+// instance's configured mode to the default — the same reasoning that
+// grandfathered `spawn_atlas_servuo_path`, `cliloc_client_path` and the seven
+// stream ids (MODULE_API.md §6.5). A module owning an unprefixed settings key is
+// a grandfathering, not a pattern to copy.
+//
+// **It was also broken.** Slice 1 ported the call site
+// (`router/player/shard.controller.js`) still calling
+// `settings.isGameAccountSignupEnabled()`, which `ctx.settings` does not expose —
+// it is three functions, not the model. So `POST /player/shard/account` threw a
+// TypeError and answered 500 for every caller, and no test saw it because the
+// module's suite never reached that branch. This file is where that function now
+// lives, on the side that actually uses it.
+
+const { settings } = require('../core')
+
+const KEY = 'game_account_signup'
+
+/**
+ * The four modes, and what each means.
+ *
+ * `website` and `hybrid` are the two that accept a site-created account; `game`
+ * means accounts are made in the client and only linked here. The shard's own
+ * `SignupMode` still has the final say when the call is actually made — this is
+ * the site half of an agreement between two systems, which is exactly why it
+ * reads as UO policy rather than as site configuration.
+ */
+const MODES = ['disabled', 'website', 'hybrid', 'game']
+const OFFERS_SIGNUP = ['website', 'hybrid']
+
+/** The configured mode, or `disabled` for anything unset or unrecognised. */
+async function getMode() {
+  const value = await settings.get(KEY)
+  return MODES.includes(value) ? value : 'disabled'
+}
+
+/**
+ * Does this site offer game-account creation right now?
+ *
+ * Fails CLOSED on an unreadable setting, because `getMode` resolves an unknown
+ * value to `disabled`. Offering a form that the shard will refuse is a dead end
+ * a player cannot distinguish from a bug.
+ */
+async function isEnabled() {
+  return OFFERS_SIGNUP.includes(await getMode())
+}
+
+/** @throws if `mode` is not one of MODES — the caller validates first. */
+async function setMode(mode, updatedBy) {
+  if (!MODES.includes(mode)) throw new Error(`unknown game-signup mode "${mode}"`)
+  return settings.set(KEY, mode, updatedBy)
+}
+
+module.exports = { KEY, MODES, OFFERS_SIGNUP, getMode, isEnabled, setMode }