fix(multi-account): persist loaded account to encrypted storage on startup

loadSavedAccount() reads from legacy files (last_account.txt) but never
wrote to the encrypted multi-account storage, so the account switcher
dropdown was always empty after restart.

Now calls ensureCurrentAccountInStorage() + refreshAccountList() after
successful load so the current account appears in the dropdown.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

desktopApp/src/jvmMain/kotlin/com/vitorpamplona/amethyst/desktop/Main.kt

@@ -672,6 +672,10 @@ fun App(
             }
             val result = accountManager.loadSavedAccount()
             if (result.isSuccess) {
+                // Ensure loaded account is in multi-account storage
+                accountManager.ensureCurrentAccountInStorage()
+                accountManager.refreshAccountList()
+
                 val current = accountManager.currentAccount()
                 if (current?.signerType is com.vitorpamplona.amethyst.commons.model.account.SignerType.Remote) {
                     accountManager.startHeartbeat(scope)