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,
 		},
 	)
 }

cmd/neoirc-cli/main.go

@@ -16,6 +16,17 @@ const (
 	pollTimeout = 15
 	pollRetry   = 2 * time.Second
 	timeFormat  = "15:04"
+
+	cmdJoin    = "JOIN"
+	cmdMotd    = "MOTD"
+	cmdNick    = "NICK"
+	cmdNotice  = "NOTICE"
+	cmdPart    = "PART"
+	cmdPrivmsg = "PRIVMSG"
+	cmdQuit    = "QUIT"
+	cmdTopic   = "TOPIC"
+	cmdWho     = "WHO"
+	cmdWhois   = "WHOIS"
 )
 
 // App holds the application state.
@@ -86,7 +97,7 @@ func (a *App) handleInput(text string) {
 	}
 
 	err := a.client.SendMessage(&api.Message{ //nolint:exhaustruct
-		Command: "PRIVMSG",
+		Command: cmdPrivmsg,
 		To:      target,
 		Body:    []string{text},
 	})
@@ -241,7 +252,7 @@ func (a *App) cmdNick(nick string) {
 	}
 
 	err := a.client.SendMessage(&api.Message{ //nolint:exhaustruct
-		Command: "NICK",
+		Command: cmdNick,
 		Body:    []string{nick},
 	})
 	if err != nil {
@@ -376,7 +387,7 @@ func (a *App) cmdMsg(args string) {
 	}
 
 	err := a.client.SendMessage(&api.Message{ //nolint:exhaustruct
-		Command: "PRIVMSG",
+		Command: cmdPrivmsg,
 		To:      target,
 		Body:    []string{text},
 	})
@@ -434,7 +445,7 @@ func (a *App) cmdTopic(args string) {
 
 	if args == "" {
 		err := a.client.SendMessage(&api.Message{ //nolint:exhaustruct
-			Command: "TOPIC",
+			Command: cmdTopic,
 			To:      target,
 		})
 		if err != nil {
@@ -447,7 +458,7 @@ func (a *App) cmdTopic(args string) {
 	}
 
 	err := a.client.SendMessage(&api.Message{ //nolint:exhaustruct
-		Command: "TOPIC",
+		Command: cmdTopic,
 		To:      target,
 		Body:    []string{args},
 	})
@@ -535,7 +546,7 @@ func (a *App) cmdMotd() {
 	}
 
 	err := a.client.SendMessage(
-		&api.Message{Command: "MOTD"}, //nolint:exhaustruct
+		&api.Message{Command: cmdMotd}, //nolint:exhaustruct
 	)
 	if err != nil {
 		a.ui.AddStatus(fmt.Sprintf(
@@ -572,7 +583,7 @@ func (a *App) cmdWho(args string) {
 
 	err := a.client.SendMessage(
 		&api.Message{ //nolint:exhaustruct
-			Command: "WHO", To: channel,
+			Command: cmdWho, To: channel,
 		},
 	)
 	if err != nil {
@@ -603,7 +614,7 @@ func (a *App) cmdWhois(args string) {
 
 	err := a.client.SendMessage(
 		&api.Message{ //nolint:exhaustruct
-			Command: "WHOIS", To: args,
+			Command: cmdWhois, To: args,
 		},
 	)
 	if err != nil {
@@ -653,7 +664,7 @@ func (a *App) cmdQuit() {
 
 	if a.connected && a.client != nil {
 		_ = a.client.SendMessage(
-			&api.Message{Command: "QUIT"}, //nolint:exhaustruct
+			&api.Message{Command: cmdQuit}, //nolint:exhaustruct
 		)
 	}
 
@@ -738,19 +749,19 @@ func (a *App) handleServerMessage(msg *api.Message) {
 	a.mu.Unlock()
 
 	switch msg.Command {
-	case "PRIVMSG":
+	case cmdPrivmsg:
 		a.handlePrivmsgEvent(msg, timestamp, myNick)
-	case "JOIN":
+	case cmdJoin:
 		a.handleJoinEvent(msg, timestamp)
-	case "PART":
+	case cmdPart:
 		a.handlePartEvent(msg, timestamp)
-	case "QUIT":
+	case cmdQuit:
 		a.handleQuitEvent(msg, timestamp)
-	case "NICK":
+	case cmdNick:
 		a.handleNickEvent(msg, timestamp, myNick)
-	case "NOTICE":
+	case cmdNotice:
 		a.handleNoticeEvent(msg, timestamp)
-	case "TOPIC":
+	case cmdTopic:
 		a.handleTopicEvent(msg, timestamp)
 	default:
 		a.handleDefaultEvent(msg, timestamp)