Fix stale docs claiming web persistence is in-memory only

Web actually persists via localStorage (LocalStorageStores.kt), which
survives page reloads — only SQLDelight itself isn't used there, since
its web driver needs a worker plus a wasm sqlite binary. The doc
comments and AGENTS.md still described the in-memory state this
replaced.

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

shared/src/commonMain/kotlin/xyz/tyiu/satsprice/data/db/SqlDelightStores.kt

@@ -10,8 +10,8 @@ import kotlin.time.Instant
 /**
  * Shared by every platform that opens a real [AppDatabase] (Android, Desktop, iOS/macOS) — only
  * driver construction differs per platform, so that's the only expect/actual boundary needed.
- * Web isn't a shipped platform (see README's Supported Platforms) and gets an in-memory fallback
- * instead of a real driver.
+ * Web has no real SQLDelight driver and uses a `localStorage`-backed fallback instead (see
+ * `LocalStorageStores.kt`).
  */
 internal class SqlDelightExchangeRateStore(private val database: AppDatabase) : ExchangeRateStore {
     override suspend fun loadLastKnownRates(sourceId: String): ExchangeRates? = withContext(Dispatchers.Default) {