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/toast.go

@@ -0,0 +1,68 @@
+package ui
+
+import (
+	"fmt"
+	"time"
+
+	tea "charm.land/bubbletea/v2"
+)
+
+// toastKind selects which of the theme's toast styles a notification uses.
+type toastKind int
+
+const (
+	toastInfo toastKind = iota
+	toastSuccess
+	toastWarn
+	toastErr
+)
+
+// toast is a transient status/error message shown in the footer. Replaces
+// the old Model.statusText/errText, which never expired once set.
+type toast struct {
+	kind toastKind
+	text string
+	seq  int
+}
+
+// toastExpireMsg clears the toast identified by seq — guarded so a
+// newer toast issued while an older one's timer is still running can't be
+// clobbered by the older timer firing after it.
+type toastExpireMsg struct{ seq int }
+
+// notify replaces the current toast and returns the tea.Cmd that expires
+// it. Errors linger noticeably longer than routine status updates.
+func (m *Model) notify(kind toastKind, format string, args ...any) tea.Cmd {
+	m.toastSeq++
+	seq := m.toastSeq
+	m.toast = &toast{kind: kind, text: fmt.Sprintf(format, args...), seq: seq}
+
+	d := 4 * time.Second
+	if kind == toastErr {
+		d = 8 * time.Second
+	}
+	return tea.Tick(d, func(time.Time) tea.Msg { return toastExpireMsg{seq: seq} })
+}
+
+func (m *Model) clearExpiredToast(msg toastExpireMsg) {
+	if m.toast != nil && m.toast.seq == msg.seq {
+		m.toast = nil
+	}
+}
+
+// toastView renders the current toast, or "" if there is none.
+func (m *Model) toastView() string {
+	if m.toast == nil {
+		return ""
+	}
+	switch m.toast.kind {
+	case toastSuccess:
+		return m.styles.ToastSuccess.Render(symOK + " " + m.toast.text)
+	case toastWarn:
+		return m.styles.ToastWarn.Render(symWarn + " " + m.toast.text)
+	case toastErr:
+		return m.styles.ToastError.Render(symErr + " " + m.toast.text)
+	default:
+		return m.styles.ToastInfo.Render(m.toast.text)
+	}
+}