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.

cmd/vault-tui/main.go

@@ -0,0 +1,22 @@
+// Command vault-tui is a terminal UI and CLI for HashiCorp Vault.
+package main
+
+import (
+	"context"
+	"fmt"
+	"os"
+
+	"git.morlana.online/f.weber/vault-tui/internal/cli"
+	_ "git.morlana.online/f.weber/vault-tui/internal/ui" // registers the "ui" command's runner
+)
+
+// version is overridden at build time via -ldflags "-X main.version=...".
+var version = "dev"
+
+func main() {
+	root := cli.Root(version)
+	if err := root.Run(context.Background(), os.Args); err != nil {
+		fmt.Fprintln(os.Stderr, "error:", err)
+		os.Exit(1)
+	}
+}