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/cli/logout.go

@@ -0,0 +1,46 @@
+package cli
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"github.com/urfave/cli/v3"
+
+	"git.morlana.online/f.weber/vault-tui/internal/token"
+)
+
+func logoutCommand() *cli.Command {
+	return &cli.Command{
+		Name:  "logout",
+		Usage: "forget the saved token (does not revoke it in Vault unless -revoke is given)",
+		Flags: []cli.Flag{
+			&cli.BoolFlag{Name: "revoke", Usage: "also revoke the token in Vault"},
+		},
+		Action: func(ctx context.Context, cmd *cli.Command) error {
+			a, err := appFrom(ctx)
+			if err != nil {
+				return err
+			}
+			revoke := cmd.Bool("revoke")
+			if p := a.File.Profiles[a.Settings.Profile]; p != nil && p.Token.RevokeOnLogout != nil {
+				revoke = revoke || *p.Token.RevokeOnLogout
+			}
+			if revoke {
+				if _, err := a.EnsureLoggedIn(ctx, cmd.String("token")); err != nil {
+					fmt.Fprintf(os.Stderr, "warning: could not verify token before revoking: %v\n", err)
+				}
+				if err := token.RevokeSelf(ctx, a.Client, a.Store); err != nil {
+					return fmt.Errorf("revoking token: %w", err)
+				}
+				fmt.Println("Token revoked and forgotten.")
+				return nil
+			}
+			if err := a.Store.Erase(ctx); err != nil {
+				return fmt.Errorf("erasing token from %s: %w", a.Store.Location(), err)
+			}
+			fmt.Println("Token forgotten (not revoked in Vault).")
+			return nil
+		},
+	}
+}