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/run.go

@@ -0,0 +1,38 @@
+// Package ui is vault-tui's Bubbletea v2 terminal UI. It talks to Vault
+// exclusively through internal/vault.Service and internal/auth.Method —
+// never hashicorp/vault/api directly — which is what keeps this package
+// swappable/testable independently of the headless CLI in internal/cli
+// (which builds the very same Service/Method values from the same
+// resolved config.Settings).
+package ui
+
+import (
+	"context"
+	"fmt"
+
+	tea "charm.land/bubbletea/v2"
+
+	"git.morlana.online/f.weber/vault-tui/internal/cli"
+)
+
+// Run launches the TUI. Wired into internal/cli's "ui" command (and its
+// DefaultCommand) via cli.SetUIRunner in cmd/vault-tui/main.go, so that
+// internal/cli itself never has to import a TUI toolkit.
+func Run(ctx context.Context, a *cli.App) error {
+	m, err := newModel(ctx, a)
+	if err != nil {
+		return err
+	}
+	p := tea.NewProgram(m, tea.WithContext(ctx))
+	_, err = p.Run()
+	return err
+}
+
+func init() {
+	cli.SetUIRunner(func(ctx context.Context, a *cli.App) error {
+		if err := Run(ctx, a); err != nil {
+			return fmt.Errorf("ui: %w", err)
+		}
+		return nil
+	})
+}