feat: update shared directories at runtime

Client::set_shared_directories rescans the given roots into a fresh
in-memory index, swaps it for peers, and re-announces the folder/file
counts to the server. The current directory list moves into the client
context so the running set is queryable. e2e-verified: a browser sees a
directory that was added after login.

soulseek-rs-lib/src/client/rooms.rs

@@ -28,7 +28,7 @@ impl Client {
 
     /// Send a raw server message via the server actor, mapping a dead channel
     /// to [`SoulseekRs::NotConnected`].
-    fn send_server_message(
+    pub(super) fn send_server_message(
         &self,
         message: crate::message::Message,
     ) -> Result<()> {