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.go

@@ -70,8 +70,10 @@ func FromTheme(theme map[string][]string) (Styles, color.Color, error) {
 			st.BorderBlurred = v.fg(lipgloss.NewStyle())
 		case "selectedLineBgColor":
 			st.Selected = v.bg(lipgloss.NewStyle()).Bold(true)
+			st.themed = true
 		case "inactiveViewSelectedLineBgColor":
 			st.SelectedBlurred = v.bg(lipgloss.NewStyle()).Bold(true)
+			st.themedBlurred = true
 		case "defaultFgColor":
 			fg = v.color
 		case "optionsTextColor":