P1: the contact list can live only on your devices

Session.roster() reads the vault once the list is off-server, mirrors
the relay's copy into the vault while one exists, and falls back to the
sealed copy when the network fails. The roster rides the history backup.
Settings gains the one-way door with the costs stated where the button
is: presence goes dark, and a new device restores contacts from the
encrypted backup rather than the server.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -104,6 +104,11 @@ class MessengerApi(
         delete("buddies/$userId")
     }
 
+    /** P1's one-way door: delete this account's contact rows, all of them. */
+    fun forgetRoster() {
+        delete("buddies")
+    }
+
     fun setStatus(status: String) {
         post("status", JSONObject().put("status", status))
     }