feat: the selected row is a shade of the terminal's background

lazysecrets asks the terminal for its background and shades the
selection from it, a little lighter on a dark background and darker on
a light one, where palette 8 was often too bright to read coloured rows
on. Palette 8 stays until the terminal answers, or if it doesn't, and
a theme's selection colours still win.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

internal/ui/styles/theme_test.go

@@ -1,6 +1,7 @@
 package styles
 
 import (
+	"fmt"
 	"strings"
 	"testing"
 
@@ -62,3 +63,32 @@ func TestEmptyThemeIsDefault(t *testing.T) {
 		t.Errorf("empty theme: %v %v reverse=%v bg=%v", err, fg, st.Selected.GetReverse(), st.Selected.GetBackground())
 	}
 }
+
+func TestOnBackground(t *testing.T) {
+	hex := func(c any) string {
+		r, g, b, _ := c.(interface{ RGBA() (r, g, b, a uint32) }).RGBA()
+		return fmt.Sprintf("#%02x%02x%02x", r>>8, g>>8, b>>8)
+	}
+	st := Default().OnBackground(lipgloss.Color("#282c34"), true)
+	if got := hex(st.Selected.GetBackground()); got != "#3e424a" {
+		t.Errorf("dark: selected %s", got)
+	}
+	if got := hex(st.SelectedBlurred.GetBackground()); got != "#343840" {
+		t.Errorf("dark: blurred %s", got)
+	}
+	if got := hex(Default().OnBackground(lipgloss.Color("#ffffff"), false).Selected.GetBackground()); got != "#e8e8e8" {
+		t.Errorf("light: selected %s", got)
+	}
+
+	themed, _, err := FromTheme(map[string][]string{"selectedLineBgColor": {"#2d3f76"}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	st = themed.OnBackground(lipgloss.Color("#282c34"), true)
+	if got := hex(st.Selected.GetBackground()); got != "#2d3f76" {
+		t.Errorf("themed: selected %s (want the theme's)", got)
+	}
+	if got := hex(st.SelectedBlurred.GetBackground()); got != "#343840" {
+		t.Errorf("themed: blurred %s (not themed: shaded)", got)
+	}
+}