feat(log): add colored terminal output without external packages

Adds ANSI color-coded log levels for terminal output:
- TRACE: Gray
- DEBUG: Cyan
- INFO: Green
- NOTICE: Blue
- WARN: Yellow
- ERROR/PANIC/FATAL: Red (bold for PANIC/FATAL)

Colors are auto-detected based on terminal capability and can be
controlled via SetColorEnabled()/ColorEnabled().

Fixes #9

README.md

@@ -15,6 +15,7 @@ A real-time log viewer with WebSocket support and namespace filtering, written i
 - **Namespace filtering**: Subscribe to specific namespaces via WebSocket
 - **Frontend namespace selector**: Filter logs by namespace in the web UI
 - **Namespace API**: GET `/api/namespaces` to list all active namespaces
+- **Colored terminal output**: Log levels are color-coded in terminal (no external packages)
 
 ## Features
 
@@ -188,6 +189,40 @@ ws://localhost:8080/ws?namespaces=api,database  # Multiple namespaces
 - **Color Coding**: Different log levels are color-coded
 - **Reconnect**: Reconnect WebSocket with new namespace filter
 
+## Terminal Colors
+
+Log output to stderr is automatically colorized when writing to a terminal. Colors are disabled when output is piped or redirected to a file.
+
+### Color Scheme
+
+| Level  | Color        |
+|--------|--------------|
+| TRACE  | Gray         |
+| DEBUG  | Cyan         |
+| INFO   | Green        |
+| NOTICE | Blue         |
+| WARN   | Yellow       |
+| ERROR  | Red          |
+| PANIC  | Bold Red     |
+| FATAL  | Bold Red     |
+
+### Controlling Colors
+
+```go
+// Disable colors (e.g., for CI/CD or file output)
+logger.SetColorEnabled(false)
+
+// Enable colors explicitly
+logger.SetColorEnabled(true)
+
+// Check current state
+if logger.ColorEnabled() {
+    // colors are on
+}
+```
+
+Colors are implemented using standard ANSI escape codes with no external dependencies.
+
 ## Migration from v1
 
 ### Import Path

log/color.go

@@ -0,0 +1,99 @@
+package log
+
+import (
+	"os"
+	"sync"
+)
+
+// ANSI color codes for terminal output
+const (
+	colorReset  = "\033[0m"
+	colorRed    = "\033[31m"
+	colorGreen  = "\033[32m"
+	colorYellow = "\033[33m"
+	colorBlue   = "\033[34m"
+	colorPurple = "\033[35m"
+	colorCyan   = "\033[36m"
+	colorWhite  = "\033[37m"
+	colorGray   = "\033[90m"
+
+	// Bold variants
+	colorBoldRed    = "\033[1;31m"
+	colorBoldYellow = "\033[1;33m"
+	colorBoldWhite  = "\033[1;37m"
+)
+
+var (
+	colorEnabled     = true
+	colorEnabledOnce sync.Once
+	colorMux         sync.RWMutex
+)
+
+// SetColorEnabled enables or disables colored output for stderr logging.
+// By default, color is enabled when stderr is a terminal.
+func SetColorEnabled(enabled bool) {
+	colorMux.Lock()
+	colorEnabled = enabled
+	colorMux.Unlock()
+}
+
+// ColorEnabled returns whether colored output is currently enabled.
+func ColorEnabled() bool {
+	colorMux.RLock()
+	defer colorMux.RUnlock()
+	return colorEnabled
+}
+
+// isTerminal checks if the given file descriptor is a terminal.
+// This is a simple heuristic that works on Unix-like systems.
+func isTerminal(f *os.File) bool {
+	stat, err := f.Stat()
+	if err != nil {
+		return false
+	}
+	return (stat.Mode() & os.ModeCharDevice) != 0
+}
+
+// initColorEnabled sets the default color state based on whether stderr is a terminal.
+func initColorEnabled() {
+	colorEnabledOnce.Do(func() {
+		colorEnabled = isTerminal(os.Stderr)
+	})
+}
+
+// levelColor returns the ANSI color code for a given log level.
+func levelColor(level Level) string {
+	switch level {
+	case LTrace:
+		return colorGray
+	case LDebug:
+		return colorCyan
+	case LInfo:
+		return colorGreen
+	case LNotice:
+		return colorBlue
+	case LWarn:
+		return colorYellow
+	case LError:
+		return colorRed
+	case LPanic:
+		return colorBoldRed
+	case LFatal:
+		return colorBoldRed
+	default:
+		return colorReset
+	}
+}
+
+// colorize wraps text with ANSI color codes if color is enabled.
+func colorize(text string, color string) string {
+	if !ColorEnabled() {
+		return text
+	}
+	return color + text + colorReset
+}
+
+// colorizeLevelText returns the level string with appropriate color.
+func colorizeLevelText(level string, lvl Level) string {
+	return colorize(level, levelColor(lvl))
+}

log/log.go

@@ -22,6 +22,7 @@ var (
 
 func init() {
 	namespaces = make(map[string]bool)
+	initColorEnabled()
 	stderrClient = CreateClient(DefaultNamespace)
 	stderrClient.SetLogLevel(LTrace)
 	stderrFinished = make(chan bool, 1)
@@ -31,7 +32,10 @@ func init() {
 func (c *Client) logStdErr() {
 	for e := range c.writer {
 		if e.level >= c.LogLevel && c.matchesNamespace(e.Namespace) {
-			fmt.Fprintf(os.Stderr, "%s\t%s\t[%s]\t%s\t%s\n", e.Timestamp.String(), e.Level, e.Namespace, e.Output, e.File)
+			levelStr := colorizeLevelText(e.Level, e.level)
+			nsStr := colorize("["+e.Namespace+"]", colorPurple)
+			fileStr := colorize(e.File, colorGray)
+			fmt.Fprintf(os.Stderr, "%s\t%s\t%s\t%s\t%s\n", e.Timestamp.String(), levelStr, nsStr, e.Output, fileStr)
 		}
 	}
 	stderrFinished <- true