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.

README.md

@@ -606,6 +606,7 @@ file, in that order of precedence. Flags work before or after the subcommand.
 | `--listener-port`              | `SOULSEEK_LISTENER_PORT`            | `listener_port`              | `2234`                    |
 | `--no-listener` / `--listener` | `SOULSEEK_NO_LISTENER`              | `disable_listener`           | listener on               |
 | —                              | —                                   | `accept_children`            | off                       |
+| —                              | —                                   | `use_username_dirs`          | off                       |
 | `--max-concurrent-downloads`   | `SOULSEEK_MAX_CONCURRENT_DOWNLOADS` | `max_concurrent_downloads`   | `20`                      |
 | `--search-timeout`             | `SOULSEEK_SEARCH_TIMEOUT`           | `search_timeout`             | `10`                      |
 | `--daemon ADDR`                | `SOULSEEK_DAEMON`                   | `daemon`                     | local socket if one is up |
@@ -627,10 +628,10 @@ to them. It is off by default — each child costs a socket and a copy of the
 network's whole search stream — and it needs the listener, since a child has to
 be able to dial in.
 
-`config get` and `config set` cover the twelve settings the file holds
+`config get` and `config set` cover the thirteen settings the file holds
 (`username`, `server`, `listener_port`, `disable_listener`, `accept_children`,
-`download_dir`, `shared_dirs`, `max_concurrent_downloads`, `search_timeout`,
-`password_cmd`, `daemon`, `daemon_token`), and list them back at you when you
+`download_dir`, `use_username_dirs`, `shared_dirs`, `max_concurrent_downloads`,
+`search_timeout`, `password_cmd`, `daemon`, `daemon_token`), and list them back at you when you
 name something
 else. An empty string clears a key, and `shared_dirs` takes a comma-separated
 list. Waits expressed in seconds (`--search-timeout`, `--timeout`,