feat: cancel a download from the TUI, the CLI, and the daemon

Closes #54.

A download can now be cancelled whether it is queued, in progress, or
paused. Cancelled is a terminal status: the store refuses to overwrite
it, the status channel relays nothing after it, the partial file is
deleted, and the transfer thread releases its socket at its next
one-second poll instead of the 30s read deadline. A queued download that
is cancelled declines the peer's later offer with TransferResponse
"Cancelled", and an inbound file connection for it is refused. A fresh
download of the same file evicts the dead row so it is not shadowed,
and a cancelled transfer is not re-queued after a restart.

Surfaces: Client::cancel_download, download.cancel over RPC, `transfer
cancel <user> <path>` on the CLI (which also stops an upload by that
name), and x in the transfers pane, where d dismisses the row.

soulseek-rs/src/SKILL.md

@@ -108,6 +108,11 @@ per finished transfer: `user`, `path`, `size`, `file` (the local path written).
 `<file>.part` behind; re-running the same command resumes from it rather than
 starting over, so retrying a failed download is cheap.
 
+**`transfer cancel <user> <path>`** — stop a download (queued or in progress;
+its `<file>.part` is deleted) or an upload being served. Prints nothing; exit 0
+is the confirmation, exit 4 means nothing by that name was in flight. Only
+useful against a daemon, since a one-shot `download` ends with its own run.
+
 **`get <query>`** — search, pick, and download in one step. `--pick
 best|first|all`, `-n/--limit`, plus every filter `search` accepts. Use it when
 any copy will do. It ranks on free slot, bitrate and peer speed, so it cannot