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/models/mod.rs

@@ -2,6 +2,7 @@ mod app_state;
 mod browse;
 mod file_display_data;
 mod rooms;
+mod settings;
 
 pub use app_state::{
     AppState, ChatMessage, CommandBarMode, DownloadEntry, FocusedPane,
@@ -12,3 +13,4 @@ pub use browse::{
 };
 pub use file_display_data::FileDisplayData;
 pub use rooms::{RoomLine, RoomsState, RoomsView};
+pub use settings::{SettingsAction, SettingsMode, SettingsState};