feat: settings popup and uploads in the transfers pane

Press 'o' for a settings popup: edit the download folder, add/remove
share paths (applied live via a rescan and persisted to config.toml),
and force a re-index with 'r' for files changed on disk. The state
machine is unit-tested; feedback shows the shared file/folder counts.

The downloads pane is now Downloads/Uploads: uploads being served are
listed after downloads with an up-arrow marker, live progress, and
status; 'x' cancels an in-progress upload. Client::shared_counts exposes
the index size for the status line.

soulseek-rs/src/ui/main_tui/mod.rs

@@ -4,6 +4,7 @@ mod input;
 mod render;
 mod rooms;
 mod search;
+mod settings;
 
 use crate::models::AppState;
 use crate::persist::{
@@ -177,6 +178,9 @@ impl MainTui {
             // Poll for chat-room events
             self.poll_room_events();
 
+            // Refresh the uploads we are serving to peers
+            self.state.uploads = self.client.uploads();
+
             self.spinner_state = (self.spinner_state + 1) % 10;
 
             self.save_persisted_state();