Add auto icon mode fallback and Nerd Font docs

internal/ui/icon_mode.go

@@ -0,0 +1,37 @@
+package ui
+
+import (
+	"os/exec"
+	"runtime"
+	"strings"
+)
+
+func resolveIconMode(mode string) (bool, string) {
+	switch strings.ToLower(strings.TrimSpace(mode)) {
+	case "ascii":
+		return false, "Icon mode: ASCII"
+	case "nerd":
+		return true, ""
+	case "", "auto":
+	default:
+		return true, ""
+	}
+
+	if runtime.GOOS != "linux" {
+		return true, ""
+	}
+	if _, err := exec.LookPath("fc-list"); err != nil {
+		return true, ""
+	}
+
+	out, err := exec.Command("fc-list", ":", "family").Output()
+	if err != nil {
+		return true, ""
+	}
+	text := strings.ToLower(string(out))
+	if strings.Contains(text, "nerd font") {
+		return true, ""
+	}
+
+	return false, "Nerd Font not found: using ASCII icons"
+}