Exclude withdrawn currencies from the picker, keep precious metals

Currency.getAvailableCurrencies() (JVM/Android) and NSLocale.ISOCurrencyCodes
(iOS/macOS) return every ISO 4217 code the platform has ever known about —
withdrawn currencies like the Deutsche Mark and French Franc, and test/
placeholder codes (XTS, XXX), included. Measured on the JVM: 232 total
codes, only 156 still assigned to a country.

Filters the list down to currencies actually assigned to an ISO country
today, which drops the withdrawn ones without needing a maintained
exclusion list. Precious metals (XAU, XAG, XPD, XPT) are added back in
since they're still actively traded despite not being tied to any country.
Web is left unfiltered — there's no reliable country-to-currency API there.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QpjMKWGoiT5aJBzhxvXkwp

shared/src/commonMain/kotlin/xyz/tyiu/satsprice/SystemCurrencies.kt

@@ -2,6 +2,14 @@ package xyz.tyiu.satsprice
 
 data class CurrencyInfo(val code: String, val displayName: String)
 
+/**
+ * ISO 4217 codes for the precious metals actively traded today. These aren't tied to any
+ * country, so a currently-used-currency filter derived from country/locale data (as the
+ * Android/JVM/Apple [systemCurrencies] implementations do, to drop long-withdrawn currencies
+ * like the Deutsche Mark) would otherwise exclude them too.
+ */
+val PRECIOUS_METAL_CURRENCY_CODES: Set<String> = setOf("XAU", "XAG", "XPD", "XPT")
+
 /** All ISO 4217 currencies the current platform knows about, with localized display names. */
 expect fun systemCurrencies(): List<CurrencyInfo>