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/src/ui/main_tui/render.rs

@@ -354,7 +354,7 @@ impl MainTui {
         let shortcuts_line = format_shortcuts_styled(&shortcuts);
         // Surface our own sharing status in the block title.
         let shared = self.client.shared_directories();
-        let sharing = match shared {
+        let sharing = match shared.as_slice() {
             [] => "off".to_string(),
             [only] => only.clone(),
             more => format!("{} folders", more.len()),