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/commonMain/kotlin/xyz/tyiu/satsprice/CurrencyFlag.kt

@@ -34,10 +34,13 @@ internal fun issuingCountryCodes(code: String): List<String> {
 
 /**
  * A country flag emoji for [code], or null when there's no flag to show — either because [code]
- * isn't tied to any country, or because it's shared by more than [MAX_FLAGS_PER_CURRENCY]
- * countries, which would be too visually noisy to show side by side.
+ * isn't tied to any country, because it's shared by more than [MAX_FLAGS_PER_CURRENCY] countries
+ * (too visually noisy to show side by side), or because [supportsFlagEmoji] says the platform
+ * can't render one anyway (Compose for Web's canvas-based text renderer has no color-emoji font
+ * to fall back to, so a flag's regional-indicator codepoints draw as empty boxes there).
  */
 fun currencyFlagEmoji(code: String): String? {
+    if (!supportsFlagEmoji()) return null
     val regionCodes = issuingCountryCodes(code)
     if (regionCodes.isEmpty() || regionCodes.size > MAX_FLAGS_PER_CURRENCY) return null
     return regionCodes.joinToString(" ") { regionFlagEmoji(it) }

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

@@ -45,3 +45,10 @@ expect fun localeCurrencyCode(): String?
 
 /** The conventional number of decimal places for [code] (e.g. 2 for USD, 0 for JPY, 3 for BHD). */
 expect fun currencyDecimalDigits(code: String): Int
+
+/**
+ * Whether the current platform's text renderer can draw flag emoji. False on Compose for Web
+ * (js/wasmJs): its canvas-based Skia renderer has no bundled or system color-emoji font to draw a
+ * flag's regional-indicator codepoints with, so they'd otherwise render as empty boxes.
+ */
+expect fun supportsFlagEmoji(): Boolean