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.

README.md

@@ -35,6 +35,8 @@ does, how to install it, and every `config.toml` setting.
 - **Browse**: list any user's shared files and download straight from the tree
 - **Resumable downloads**: a dead transfer leaves a `.part`, and re-running it
   asks the peer for only what is missing
+- **Cancel a transfer**: `x` in the TUI or `transfer cancel` from a script stops
+  a download, queued or in progress, and deletes its `.part`; uploads too
 - **Chat rooms**: list, join, and talk in public rooms, several open at once
 - **Private messages**: send and receive, with an inbox in the TUI
 - **Firewalled peers**: browsing and downloads fall back to server-brokered
@@ -251,6 +253,7 @@ soulseek-rs search <QUERY>                # print matching files
 soulseek-rs download <USER> <PATH>        # fetch one known file
 soulseek-rs download --stdin              # fetch files listed on stdin
 soulseek-rs get <QUERY>                   # search, pick, and download
+soulseek-rs transfer cancel <USER> <PATH> # stop a download or upload
 soulseek-rs wish add|remove|list <QUERY>  # keep looking for something
 soulseek-rs wish run                      # search every stored wish once
 soulseek-rs browse <USER>                 # list a user's shared files
@@ -290,9 +293,9 @@ account of its own can still ask whether one is running. `shares status` and
 `shares reindex` are not among them, because reporting what the network sees
 means logging in and scanning the folders.
 
-Six commands perform an action and print nothing, answering with their exit
+Seven commands perform an action and print nothing, answering with their exit
 code alone: `room say`, `message send`, `shares add`, `shares remove`,
-`wish remove` and `daemon stop`. `room listen`, `message read` and `serve`
+`wish remove`, `transfer cancel` and `daemon stop`. `room listen`, `message read` and `serve`
 stream records until `--duration` seconds pass or, with `--follow`, until
 interrupted. The rest print records and exit.