Registered devices card on Account — list, date, and remove

Same contract as iOS: the relay's roster for this account, oldest
first, the current install marked and not removable from itself (that
is Sign out), removal behind an in-place "Really remove" confirmation
per this screen's house style. RelayClient grows revokeDevice
(DELETE /e2e/devices/{id}); Session surfaces ownDevices()/revokeDevice
and parses the directory's new created_at.

The relay caps accounts at three devices, so this card is the only way
off the cap when registration refuses a fourth.

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

client/src/main/kotlin/io/flamenet/client/RelayClient.kt

@@ -108,6 +108,14 @@ class RelayClient(
     fun listDevices(userId: Long): JSONArray =
         getJson("/e2e/devices/$userId").optJSONArray("devices") ?: JSONArray()
 
+    /**
+     * `DELETE /e2e/devices/{id}` — remove one of the ACCOUNT'S OWN devices.
+     * The relay refuses anyone else's, and drops the device's prekeys and
+     * queued mail with it (§8.3a).
+     */
+    fun revokeDevice(deviceId: String): JSONObject =
+        requestJson("DELETE", url("/e2e/devices/$deviceId"), "/e2e/devices/$deviceId", null)
+
     // -- Messages ------------------------------------------------------------
 
     /** One envelope for one of the recipient's devices. */