Match currency search against issuing country names too

Searching "Japan" now finds JPY, "Canada" finds CAD, etc. — matching
just the currency's own display name wasn't enough, since e.g.
"Canadian Dollar" doesn't contain "Canada" as a substring.

Adds a per-platform regionDisplayName(regionCode) (java.util.Locale
on JVM/Android, NSLocale on Apple, Intl.DisplayNames on web) and
reuses CurrencyFlag.kt's currency-to-country-code derivation (now
extracted as issuingCountryCodes) to look up the country/countries
behind a currency code. "EU" is hardcoded to "European Union" since
it's not a real ISO 3166-1 code platform locale data can resolve.

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

shared/src/jvmMain/kotlin/xyz/tyiu/satsprice/SystemCurrencies.jvm.kt

@@ -37,3 +37,9 @@ actual fun currencyDecimalDigits(code: String): Int = try {
 } catch (e: IllegalArgumentException) {
     2
 }
+
+@Suppress("DEPRECATION") // Locale(language, country) still works fine; Locale.of() needs newer Android API levels.
+actual fun regionDisplayName(regionCode: String): String? {
+    if (regionCode !in Locale.getISOCountries()) return null
+    return Locale("", regionCode).getDisplayCountry(Locale.getDefault())
+}