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/config/paths.go

@@ -0,0 +1,52 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+)
+
+const (
+	appDirName  = "vault-tui"
+	fileName    = "config.yaml"
+	configEnv   = "VAULT_TUI_CONFIG"
+	profileEnv  = "VAULT_TUI_PROFILE"
+	noEnvEnv    = "VAULT_TUI_NO_ENV"
+	readOnlyEnv = "VAULT_TUI_READ_ONLY"
+)
+
+// DefaultPath returns the config file location used when neither --config
+// nor VAULT_TUI_CONFIG is set: $XDG_CONFIG_HOME/vault-tui/config.yaml,
+// falling back to ~/.config/vault-tui/config.yaml.
+func DefaultPath() (string, error) {
+	dir, err := os.UserConfigDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(dir, appDirName, fileName), nil
+}
+
+// ResolvePath applies the discovery order: --config flag > VAULT_TUI_CONFIG
+// env > default XDG location.
+func ResolvePath(flag string) (string, error) {
+	if flag != "" {
+		return flag, nil
+	}
+	if v := os.Getenv(configEnv); v != "" {
+		return v, nil
+	}
+	return DefaultPath()
+}
+
+// StateDir returns the directory vault-tui uses for its own per-profile
+// token cache (internal/token/store_profile.go): $XDG_STATE_HOME/vault-tui,
+// falling back to ~/.local/state/vault-tui.
+func StateDir() (string, error) {
+	if v := os.Getenv("XDG_STATE_HOME"); v != "" {
+		return filepath.Join(v, appDirName), nil
+	}
+	home, err := os.UserHomeDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(home, ".local", "state", appDirName), nil
+}