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) {

shared/src/webMain/kotlin/xyz/tyiu/satsprice/data/db/LocalStorageStores.kt

@@ -10,10 +10,11 @@ import xyz.tyiu.satsprice.data.ExchangeRates
 import kotlin.time.Instant
 
 /**
- * Web isn't a shipped platform (see README's Supported Platforms) and SQLDelight's web driver
- * needs a worker plus a wasm sqlite binary, so persistence here goes through the browser's
- * localStorage instead of a real database — plenty for this small amount of data, and it
- * survives page reloads unlike the in-memory state it replaced.
+ * SQLDelight's web driver needs a worker plus a wasm sqlite binary, so persistence here goes
+ * through the browser's localStorage instead of a real database — plenty for this small amount
+ * of data, and it survives page reloads like the other platforms' real databases do. Scoped to
+ * the browser profile/origin, though: it won't survive a cleared site data / private window, and
+ * doesn't sync or back up anywhere.
  */
 private val json = Json { ignoreUnknownKeys = true }