feat(vault-tui): implement KV client and service for managing secrets

- Added internal/vault/client.go for creating a Vault client with configuration settings.
- Introduced internal/vault/errors.go to classify Vault API errors for better UI handling.
- Created internal/vault/kv.go to manage KV secrets, including listing, reading, writing, and deleting operations.
- Implemented internal/vault/mounts.go to list and describe secret engine mounts.
- Developed internal/vault/service.go to provide a unified entry point for Vault operations.
- Added internal/vault/kv_test.go for comprehensive testing of KV operations.
- Introduced internal/ui/toast.go for transient notifications in the UI.
- Added renovate.json for dependency management and updates.

internal/ui/screen_help.go

@@ -0,0 +1,27 @@
+package ui
+
+import (
+	tea "charm.land/bubbletea/v2"
+	"charm.land/lipgloss/v2"
+)
+
+// updateHelp handles the "?" full-keybinding overlay — any key closes it,
+// since it's purely informational.
+func (m *Model) updateHelp(msg tea.KeyPressMsg) (tea.Model, tea.Cmd) {
+	m.helpOpen = false
+	return m, nil
+}
+
+func (m *Model) viewHelp() string {
+	w := m.lay.w - 12
+	if w > 72 {
+		w = 72
+	}
+	if w < 30 {
+		w = 30
+	}
+	m.help.SetWidth(w)
+	body := m.help.FullHelpView(m.keys.FullHelp())
+	lines := []string{m.styles.ModalTitle.Render("Keybindings"), "", body, "", m.styles.Help.Render("press any key to close")}
+	return m.styles.Modal.Render(lipgloss.JoinVertical(lipgloss.Left, lines...))
+}