feat(cli): update client to use unified command endpoint

- JoinChannel/PartChannel now send via POST /messages with command field
- ListChannels uses /channels instead of /channels/all
- gofmt whitespace fixes

cmd/chat-cli/main.go

@@ -15,12 +15,12 @@ type App struct {
 	ui     *UI
 	client *api.Client
 
-	mu         sync.Mutex
-	nick       string
-	target     string // current target (#channel or nick for DM)
-	connected  bool
-	lastMsgID  string
-	stopPoll   chan struct{}
+	mu        sync.Mutex
+	nick      string
+	target    string // current target (#channel or nick for DM)
+	connected bool
+	lastMsgID string
+	stopPoll  chan struct{}
 }
 
 func main() {