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-lib/src/client/mod.rs

@@ -671,6 +671,14 @@ impl Client {
             .unwrap_or_default()
     }
 
+    /// `(folders, files)` currently shared with peers.
+    #[must_use]
+    pub fn shared_counts(&self) -> (u32, u32) {
+        self.context.read_safe().map_or((0, 0), |ctx| {
+            (ctx.shares.folder_count(), ctx.shares.file_count())
+        })
+    }
+
     /// Snapshot of the uploads served this session (active and finished),
     /// most recent last.
     #[must_use]