Refactor Vue sample app for shared API logic and add targeted unit tests

[Copilot]

This PR reworks the Vue sample app to reduce duplication and improve maintainability, while adding focused test coverage around shared request/snippet behavior. It also updates package/build configuration where required to support the refactor and test execution.

• Shared request/auth utilities

  • Added js/utils/api.js for auth header creation and standardized response formatting.
  • Added js/utils/credentials.js to centralize credential verification requests.
  • Replaced repeated inline auth-header/response formatting code in feature components with utility calls.

• Snippet generation cleanup

  • Added js/utils/snippet.js to encapsulate snippet payload shaping (GET query vs POST body).
  • Updated mixins.js to delegate snippet construction to the new utility.

• Component simplification

  • Simplified App.vue and GoBack.vue by removing unused component/state boilerplate.
  • Updated CodeView.vue lifecycle hook usage to Vue 3 idioms (beforeUnmount).
  • Tightened credential validation flow in Home.vue with clearer failure messaging paths.

• Test coverage additions

  • Added unit tests for:
    • api utility behavior
    • snippet utility transformations
    • credential verification request wiring
    • mixin auth/snippet integration behavior

• Package/build updates

  • Added vitest and a test script.
  • Added browser polyfills required by dependency graph updates (crypto-browserify, vm-browserify) and wired webpack fallbacks.

// Before: each component rebuilt auth headers and response formatting inline
const headers = {
  "X-Requested-With": "XMLHttpRequest",
  Authorization: `Basic ${btoa(`${store.appId}:${store.appSecret}`)}`,
};

// After: shared utility
const headers = getAuthHeaders();
setFormattedResponse(response.data);