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.

renovate.json

@@ -0,0 +1,33 @@
+{
+  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
+  "extends": [
+    "config:recommended"
+  ],
+  "schedule": ["before 6am on monday"],
+  "timezone": "Europe/Berlin",
+  "labels": ["dependencies"],
+  "postUpdateOptions": ["gomodTidy"],
+  "packageRules": [
+    {
+      "matchManagers": ["gomod"],
+      "matchUpdateTypes": ["patch", "minor"],
+      "groupName": "go dependencies (non-major)",
+      "automerge": false
+    },
+    {
+      "matchManagers": ["gomod"],
+      "matchUpdateTypes": ["major"],
+      "groupName": "go dependencies (major)",
+      "automerge": false
+    },
+    {
+      "matchManagers": ["github-actions"],
+      "groupName": "actions",
+      "automerge": false
+    }
+  ],
+  "lockFileMaintenance": {
+    "enabled": true,
+    "schedule": ["before 6am on monday"]
+  }
+}