Hide flag emoji on the web target, where they render as empty boxes

Compose for Web (js/wasmJs) draws everything through Skia onto a
<canvas> rather than through the browser's own text stack, so it has
no color-emoji font to fall back to — a flag's regional-indicator
codepoints render as empty tofu boxes instead of a flag. Reproduced
via a headless-Chrome screenshot of the running dev server.

Adds a per-platform supportsFlagEmoji() (true everywhere except
js/wasmJs) that currencyFlagEmoji() now checks first, so currency
rows on web just show the plain code/name with no flag rather than a
broken glyph.

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

@@ -43,3 +43,5 @@ actual fun regionDisplayName(regionCode: String): String? {
     if (regionCode !in Locale.getISOCountries()) return null
     return Locale("", regionCode).getDisplayCountry(Locale.getDefault())
 }
+
+actual fun supportsFlagEmoji(): Boolean = true