fix: register /cancel as a proper command

Previously /cancel was intercepted inside the 'unknown' branch, and the
comparison used the command WITHOUT the leading slash while parseCommand
returns it WITH one ('/cancel') — the check never matched, so /cancel
always fell through to the unknown-command reply, even with a pending
ask/approval.

Now a first-class command (menu included): closes the TG-side
answer/approval channel when one is pending, otherwise reports that
nothing is awaiting a reply.

lib/commands.d.ts

@@ -53,6 +53,9 @@ export type ParsedCommand = {
 } | {
     type: 'new';
     text: string;
+} | {
+    type: 'cancel';
+    text: string;
 } | {
     type: 'unknown';
     command: string;