refactor: replace all bare command string literals with named constants

Extract cmdLusers, cmdMode, cmdMotd, cmdNames, cmdNotice, cmdPing, cmdPong
constants in internal/handlers/api.go. Add corresponding constants in
cmd/neoirc-cli/main.go and cmd/neoirc-cli/api/client.go. Replace every bare
IRC command string literal in switch cases and command dispatch code with the
named constant. Zero bare command strings remain in any dispatch path.

cmd/neoirc-cli/api/client.go

@@ -19,6 +19,9 @@ const (
 	httpTimeout      = 30 * time.Second
 	pollExtraTime    = 5
 	httpErrThreshold = 400
+
+	cmdJoin = "JOIN"
+	cmdPart = "PART"
 )
 
 var errHTTP = errors.New("HTTP error")
@@ -168,7 +171,7 @@ func (client *Client) PollMessages(
 func (client *Client) JoinChannel(channel string) error {
 	return client.SendMessage(
 		&Message{ //nolint:exhaustruct // only command+to needed
-			Command: "JOIN", To: channel,
+			Command: cmdJoin, To: channel,
 		},
 	)
 }
@@ -177,7 +180,7 @@ func (client *Client) JoinChannel(channel string) error {
 func (client *Client) PartChannel(channel string) error {
 	return client.SendMessage(
 		&Message{ //nolint:exhaustruct // only command+to needed
-			Command: "PART", To: channel,
+			Command: cmdPart, To: channel,
 		},
 	)
 }