feat: multiple shared directories

Shares::scan_many merges several roots into one index, suffixing
duplicate root names so virtual paths cannot collide, and skipping
unreadable roots instead of failing. ClientSettings now takes
shared_directories: Vec<String>; config gains a shared_dirs list
(combined with the single shared_dir and --shared-dir, deduped), with an
explicitly empty value still disabling sharing and the
share-what-you-download default unchanged.

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

@@ -353,10 +353,13 @@ impl MainTui {
 
         let shortcuts_line = format_shortcuts_styled(&shortcuts);
         // Surface our own sharing status in the block title.
-        let title = format!(
-            "Shortcuts · Sharing: {}",
-            self.client.shared_directory().unwrap_or("off")
-        );
+        let shared = self.client.shared_directories();
+        let sharing = match shared {
+            [] => "off".to_string(),
+            [only] => only.clone(),
+            more => format!("{} folders", more.len()),
+        };
+        let title = format!("Shortcuts · Sharing: {sharing}");
         let shortcuts_widget = Paragraph::new(shortcuts_line).block(
             Block::default()
                 .borders(Borders::ALL)