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/write.go

@@ -0,0 +1,69 @@
+package cli
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/urfave/cli/v3"
+)
+
+func writeCommand() *cli.Command {
+	return &cli.Command{
+		Name:      "write",
+		Usage:     "create or update a secret (e.g. `vault-tui write secret/team/prod/db user=admin pass=hunter2`)",
+		ArgsUsage: "<path> key=value [key=value ...]",
+		Flags: []cli.Flag{
+			&cli.IntFlag{Name: "cas", Usage: "KV v2 check-and-set version (default: current version, if require_cas is on)"},
+			&cli.BoolFlag{Name: "force", Usage: "skip check-and-set even if require_cas is on"},
+		},
+		Action: func(ctx context.Context, cmd *cli.Command) error {
+			a, err := appFrom(ctx)
+			if err != nil {
+				return err
+			}
+			args := cmd.Args().Slice()
+			if len(args) < 2 {
+				return fmt.Errorf("usage: vault-tui write <path> key=value [key=value ...]")
+			}
+			if _, err := a.EnsureLoggedIn(ctx, cmd.String("token")); err != nil {
+				return err
+			}
+			svc := a.Service()
+			if svc.ReadOnly {
+				return fmt.Errorf("refusing to write: vault-tui is in read-only mode (pass --write to override)")
+			}
+			mount, rel, err := splitMountPath(ctx, svc, args[0])
+			if err != nil {
+				return err
+			}
+			data, err := parseKVArgs(args[1:])
+			if err != nil {
+				return err
+			}
+			iface := make(map[string]interface{}, len(data))
+			for k, v := range data {
+				iface[k] = v
+			}
+
+			useCAS := a.Settings.RequireCAS && !cmd.Bool("force")
+			cas := int(cmd.Int("cas"))
+			if useCAS && !cmd.IsSet("cas") {
+				current, err := svc.KV.Read(ctx, mount, rel, 0)
+				if err == nil && current != nil {
+					cas = current.Version
+				}
+			}
+
+			ack, err := svc.KV.Write(ctx, mount, rel, iface, useCAS, cas)
+			if err != nil {
+				return err
+			}
+			if ack.Version > 0 {
+				fmt.Printf("wrote %s (version %d)\n", args[0], ack.Version)
+			} else {
+				fmt.Printf("wrote %s\n", args[0])
+			}
+			return nil
+		},
+	}
+}