Use the browser's own locale for web currency display names

jsCurrencyDisplayName() was hardcoding 'en' for Intl.DisplayNames,
so every currency name shown on web (USD, JPY, ...) stayed English
regardless of the user's locale — same class of bug as the "EU"
string, just for every currency rather than one region. Switched to
undefined, matching regionDisplayName()'s fix; verified with Node
that this actually produces locale-appropriate names (e.g. "US
Dollar" / "dollar des États-Unis" / "米ドル").

Left jsCurrencyFractionDigits()'s 'en' alone and documented why:
verified with Node that a currency's decimal-digit count doesn't
vary by locale, so locale there is just a required constructor
argument, not something the result depends on.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FXcrACM5sok3M1KQJ8kByy

shared/src/webMain/kotlin/xyz/tyiu/satsprice/SystemCurrencies.web.kt

@@ -6,8 +6,10 @@ private fun jsSupportedCurrencyCodes(): JsArray<JsString> = js("Intl.supportedVa
 
 // A chained `new Foo(x).bar()` inside js() can misparse (`new` binding to the whole chain rather
 // than just the constructor call), so the constructor result is bound to a variable first.
+// `undefined` (rather than a fixed locale like 'en') uses the browser's own locale — code always
+// comes from jsSupportedCurrencyCodes() itself, so .of(code) is guaranteed to resolve.
 private fun jsCurrencyDisplayName(code: String): String = js(
-    "(function() { var names = new Intl.DisplayNames(['en'], { type: 'currency' }); return names.of(code); })()",
+    "(function() { var names = new Intl.DisplayNames(undefined, { type: 'currency' }); return names.of(code); })()",
 )
 
 actual fun systemCurrencies(): List<CurrencyInfo> =
@@ -27,6 +29,10 @@ actual fun systemCurrencies(): List<CurrencyInfo> =
  */
 actual fun localeCurrencyCode(): String? = null
 
+// A currency's decimal-digit count is a property of the currency, not the locale (e.g. JPY's is
+// always 0), so — unlike jsCurrencyDisplayName/jsRegionDisplayName above — locale is only a
+// required constructor argument here, not something the result actually depends on; 'en' is as
+// good as any other.
 private fun jsCurrencyFractionDigits(code: String): Int = js(
     """(function() { var fmt = new Intl.NumberFormat('en', { style: 'currency', currency: code }); return fmt.resolvedOptions().maximumFractionDigits; })()""",
 )