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/color.go

@@ -0,0 +1,40 @@
+package config
+
+import (
+	"fmt"
+
+	"gopkg.in/yaml.v3"
+)
+
+// UnmarshalYAML accepts either a scalar ("#7D56F4") applied to both
+// appearances, or a mapping ({light: "#...", dark: "#..."}).
+func (c *Color) UnmarshalYAML(n *yaml.Node) error {
+	switch n.Kind {
+	case yaml.ScalarNode:
+		var s string
+		if err := n.Decode(&s); err != nil {
+			return err
+		}
+		c.Light, c.Dark = s, s
+		return nil
+	case yaml.MappingNode:
+		var pair struct {
+			Light string `yaml:"light"`
+			Dark  string `yaml:"dark"`
+		}
+		if err := n.Decode(&pair); err != nil {
+			return err
+		}
+		c.Light, c.Dark = pair.Light, pair.Dark
+		return nil
+	default:
+		return fmt.Errorf("color must be a string or a {light, dark} mapping, got %v", n.Kind)
+	}
+}
+
+func (c Color) MarshalYAML() (interface{}, error) {
+	if c.Light == c.Dark {
+		return c.Light, nil
+	}
+	return map[string]string{"light": c.Light, "dark": c.Dark}, nil
+}