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/kv_helpers.go

@@ -0,0 +1,39 @@
+package cli
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"git.morlana.online/f.weber/vault-tui/internal/vault"
+)
+
+// splitMountPath finds which configured mount a full logical path like
+// "secret/team/prod/db" belongs to, and returns the mount plus the
+// remaining path relative to it ("team/prod/db"). Mirrors how `vault kv`
+// itself resolves a bare path against sys/internal/ui/mounts.
+func splitMountPath(ctx context.Context, svc *vault.Service, full string) (vault.Mount, string, error) {
+	full = strings.TrimPrefix(full, "/")
+	mounts, err := svc.Mounts(ctx)
+	if err != nil {
+		return vault.Mount{}, "", fmt.Errorf("listing mounts: %w", err)
+	}
+	var best vault.Mount
+	for _, m := range mounts {
+		mp := strings.TrimSuffix(m.Path, "/")
+		if full == mp || strings.HasPrefix(full, mp+"/") {
+			if len(mp) > len(strings.TrimSuffix(best.Path, "/")) {
+				best = m
+			}
+		}
+	}
+	if best.Path == "" {
+		return vault.Mount{}, "", fmt.Errorf("no configured mount matches %q", full)
+	}
+	if !best.Supported() {
+		return vault.Mount{}, "", fmt.Errorf("mount %q (type %s) is not supported by vault-tui", best.Path, best.Type)
+	}
+	rel := strings.TrimPrefix(full, strings.TrimSuffix(best.Path, "/"))
+	rel = strings.TrimPrefix(rel, "/")
+	return best, rel, nil
+}