fix(tui): apply username folders everywhere and resume in place

Move the sanitizer next to the directory helpers so the CLI download
command and the daemon honour use_username_dirs too, not only the TUI.
Resumed downloads keep the folder they were saved with instead of
nesting the username a second time, and toggling the setting applies
to the next download without a restart. Document the config key.

soulseek-rs/src/run.rs

@@ -276,6 +276,7 @@ fn context(
             version: ClientVersion::REFERENCE_CLIENT,
         },
         download_dir,
+        use_username_dirs: resolved.use_username_dirs,
         max_concurrent_downloads: resolved.max_concurrent_downloads,
         search_timeout: Duration::from_secs(resolved.search_timeout),
         daemon,