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/app/app.go

@@ -489,7 +489,7 @@ func (m *Model) setVersions(items []provider.Object) {
 }
 
 func (m Model) Init() tea.Cmd {
-	cmds := []tea.Cmd{m.sourcesCmd(), m.spin.Tick, m.checkClipboard()}
+	cmds := []tea.Cmd{m.sourcesCmd(), m.spin.Tick, m.checkClipboard(), tea.RequestBackgroundColor}
 	if m.cachePath != "" || m.placeDir != "" {
 		cmds = append(cmds, tea.Tick(persistEvery, func(time.Time) tea.Msg { return persistTickMsg{} }))
 	}
@@ -514,6 +514,9 @@ func (m Model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 	var cmds []tea.Cmd
 
 	switch msg := msg.(type) {
+	case tea.BackgroundColorMsg:
+		m.st = m.st.OnBackground(msg.Color, msg.IsDark())
+		return m, nil
 	case tea.WindowSizeMsg:
 		m.w, m.h = msg.Width, msg.Height