feat: add tray layout controls

internal/config/config.go

@@ -200,6 +200,63 @@ func (c Config) LayoutFor(device string) (Layout, bool) {
 	return Layout{}, false
 }
 
+func (c Config) LayoutNames() []string {
+	seen := make(map[string]bool)
+	var names []string
+	for _, l := range c.Layouts {
+		name := strings.TrimSpace(l.Name)
+		key := strings.ToLower(name)
+		if name != "" && !seen[key] {
+			seen[key] = true
+			names = append(names, name)
+		}
+	}
+	return names
+}
+
+// SetActiveLayout updates only the top-level active_layout line, preserving the
+// user's comments, layout formatting, and ordering.
+func SetActiveLayout(path, name string) error {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return err
+	}
+	encoded, err := yaml.Marshal(strings.TrimSpace(name))
+	if err != nil {
+		return err
+	}
+	value := strings.TrimSpace(string(encoded))
+	newline := "\n"
+	if strings.Contains(string(data), "\r\n") {
+		newline = "\r\n"
+	}
+	lines := strings.Split(strings.ReplaceAll(string(data), "\r\n", "\n"), "\n")
+	replaced := false
+	for i, line := range lines {
+		if strings.HasPrefix(line, "active_layout:") {
+			lines[i] = "active_layout: " + value
+			replaced = true
+			break
+		}
+	}
+	if !replaced {
+		insertAt := 0
+		for i, line := range lines {
+			if strings.HasPrefix(line, "version:") || strings.HasPrefix(line, "gap:") || strings.HasPrefix(line, "shift_drag:") {
+				insertAt = i + 1
+			}
+		}
+		lines = append(lines, "")
+		copy(lines[insertAt+1:], lines[insertAt:])
+		lines[insertAt] = "active_layout: " + value
+	}
+	info, err := os.Stat(path)
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(path, []byte(strings.Join(lines, newline)), info.Mode().Perm())
+}
+
 func (c Config) IsExcluded(exe string) bool {
 	// Windows paths may be validated on another OS during cross-compilation.
 	normalized := strings.ReplaceAll(exe, `\`, "/")

internal/config/config_test.go

@@ -3,6 +3,7 @@ package config
 import (
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -64,6 +65,37 @@ func TestMissingActiveLayoutFails(t *testing.T) {
 	}
 }
 
+func TestLayoutNamesAreUniqueAndOrdered(t *testing.T) {
+	c := Default()
+	c.Layouts = append(c.Layouts,
+		Layout{Name: "Columns", Monitor: `\\.\DISPLAY2`},
+		Layout{Name: "Rows", Monitor: "*"},
+	)
+	got := c.LayoutNames()
+	if len(got) != 2 || got[0] != "columns" || got[1] != "Rows" {
+		t.Fatalf("unexpected names: %#v", got)
+	}
+}
+
+func TestSetActiveLayoutPreservesConfiguration(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "fancywin.yaml")
+	original := "# keep this comment\r\nversion: 1\r\nactive_layout: old # old value\r\nlayouts: []\r\n"
+	if err := os.WriteFile(path, []byte(original), 0o640); err != nil {
+		t.Fatal(err)
+	}
+	if err := SetActiveLayout(path, "new layout"); err != nil {
+		t.Fatal(err)
+	}
+	data, err := os.ReadFile(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	text := string(data)
+	if !strings.Contains(text, "# keep this comment\r\n") || !strings.Contains(text, "active_layout: new layout\r\n") || !strings.Contains(text, "layouts: []\r\n") {
+		t.Fatalf("unexpected rewritten file: %q", text)
+	}
+}
+
 func TestExcluded(t *testing.T) {
 	c := Default()
 	c.Excluded = []string{"notepad", "exact.exe"}