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/ui/textinput.go

@@ -0,0 +1,35 @@
+package ui
+
+import "charm.land/bubbles/v2/textinput"
+
+// textInput is a local alias purely to keep the many field declarations in
+// model.go from repeating the full package-qualified name.
+type textInput = textinput.Model
+
+// newTextInput is a method (not a free function) so every input picks up
+// the current theme's colors via styles.TextInputStyles — otherwise inputs
+// would render in bubbles' hard-coded default palette regardless of the
+// user's theme/appearance settings.
+func (m *Model) newTextInput(f fieldSpec) textInput {
+	ti := textinput.New()
+	ti.Placeholder = f.label
+	ti.SetStyles(m.styles.TextInputStyles())
+	if f.secret {
+		ti.EchoMode = textinput.EchoPassword
+		ti.EchoCharacter = '•'
+	}
+	ti.SetValue(f.value)
+	return ti
+}
+
+func (m *Model) textInputWithValue(label, val string) textInput {
+	return m.newTextInput(fieldSpec{label: label, value: val})
+}
+
+// fieldSpec is the minimal shape newTextInput needs; kept separate from
+// auth.Field so this file has no dependency on the auth package.
+type fieldSpec struct {
+	label  string
+	secret bool
+	value  string
+}