ci: base64 from java.util, Bytes is internal to the engine

StoryTakeDownTest reached for `io.flamenet.e2e.Bytes`, which is internal to
the engine module, so the app's test source set cannot see it. Both that
helper and the client's `Wire.b64` are `Base64.getEncoder().encodeToString`,
which is what the linked-device map is compared against, so the test uses it
directly, as the other app tests do.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

app/src/test/kotlin/io/flamenet/messenger/app/StoryTakeDownTest.kt

@@ -4,7 +4,6 @@ import io.flamenet.client.Conversations
 import io.flamenet.client.MessengerApi
 import io.flamenet.client.RelayClient
 import io.flamenet.e2e.Attachment
-import io.flamenet.e2e.Bytes
 import io.flamenet.e2e.Content
 import io.flamenet.e2e.DeviceIdentity
 import io.flamenet.e2e.E2ESession
@@ -21,6 +20,7 @@ import org.robolectric.RobolectricTestRunner
 import org.robolectric.annotation.Config
 import java.io.File
 import java.nio.file.Files
+import java.util.Base64
 import javax.crypto.KeyGenerator
 import kotlin.test.AfterTest
 import kotlin.test.BeforeTest
@@ -299,7 +299,10 @@ class StoryTakeDownTest {
         // with the password is a device of the account and nothing more, so
         // without this the tablet is registered and still gets nothing.
         vault.saveLinkedDevices(
-            mapOf(myTablet.deviceId to Bytes.b64(myTablet.identitySigningPublic)),
+            mapOf(
+                myTablet.deviceId to
+                    Base64.getEncoder().encodeToString(myTablet.identitySigningPublic),
+            ),
         )
         val story = photo(blobId(4), me, sentTo = listOf(sam))
         hold(story)