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/docs/openrpc.json

@@ -416,6 +416,18 @@
             ],
             "type": "object"
           },
+          {
+            "properties": {
+              "state": {
+                "const": "cancelled",
+                "type": "string"
+              }
+            },
+            "required": [
+              "state"
+            ],
+            "type": "object"
+          },
           {
             "properties": {
               "reason": {
@@ -1648,6 +1660,33 @@
       },
       "summary": "Drop a transfer that has not started."
     },
+    {
+      "name": "download.cancel",
+      "paramStructure": "by-name",
+      "params": [
+        {
+          "name": "filename",
+          "required": true,
+          "schema": {
+            "type": "string"
+          }
+        },
+        {
+          "name": "username",
+          "required": true,
+          "schema": {
+            "type": "string"
+          }
+        }
+      ],
+      "result": {
+        "name": "result",
+        "schema": {
+          "$ref": "#/components/schemas/Ack"
+        }
+      },
+      "summary": "Stop a download, queued or in progress; its partial file is deleted."
+    },
     {
       "name": "download.set_dir",
       "paramStructure": "by-name",