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.

website/src/pages/install.tsx

@@ -129,6 +129,11 @@ const SETTINGS: readonly Setting[] = [
       </>
     ),
   },
+  {
+    key: 'use_username_dirs',
+    fallback: 'false',
+    body: <>Save each peer's files in a subfolder named after them.</>,
+  },
   {
     key: 'max_concurrent_downloads',
     fallback: '20',