feat(utils): add persistent cache and backend error utilities

[nicomiguelino]

Summary

• Add createPersistentCache(), a localStorage-backed, namespaced last-known-good value cache
• Add BackendServerError and shouldSkipBackendError() to gate falling back to cached data on transient backend failures
• Document both utilities with usage examples in README
• Bump package version to 1.3.0

Test plan

• npm test — new unit tests for createPersistentCache and shouldSkipBackendError pass

[Copilot]

Pull request overview

Adds two new utilities to the @screenly/edge-apps utils surface area: a localStorage-backed “last-known-good” persistent cache and a small error utility (BackendServerError + shouldSkipBackendError) to control when cached fallback is allowed. This fits the library’s existing goal of providing reusable Edge App platform/helpers via the src/utils barrel.

Changes:

• Add createPersistentCache(namespace) with read/write APIs and unit tests.
• Add BackendServerError and shouldSkipBackendError() with unit tests to gate cache fallback based on display_errors.
• Document the new utilities in README.md and bump package version to 1.3.0.

Reviewed changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/utils/persistent-cache.ts	Introduces a namespaced localStorage persistent cache utility with safe (non-throwing) read/write behavior.
src/utils/persistent-cache.test.ts	Adds unit tests for cache behavior, namespacing, invalid JSON, and missing localStorage.
src/utils/index.ts	Re-exports the new backend error and persistent cache utilities from the utils barrel.
src/utils/backend-error.ts	Adds BackendServerError and shouldSkipBackendError() for “transient backend failure” classification.
src/utils/backend-error.test.ts	Adds unit tests validating shouldSkipBackendError() behavior.
README.md	Documents usage patterns for persistent cache + backend transient error gating and updates the exported types example.
package.json	Bumps package version to 1.3.0.
package-lock.json	Updates lockfile version metadata to 1.3.0.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated 5 comments.

Comments suppressed due to low confidence (2)

src/utils/persistent-cache.test.ts:47

• Deleting/restoring globalThis.localStorage can throw in some environments (non-configurable globals) and doesn't use the repo's existing stubbing pattern (vi.stubGlobal / vi.unstubAllGlobals). Consider stubbing via Vitest with a try/finally to ensure cleanup even if an assertion fails.

    const originalLocalStorage = globalThis.localStorage
    // @ts-expect-error simulating an environment without localStorage
    delete globalThis.localStorage

    const cache = createPersistentCache('test-app')
    expect(cache.read('credentials')).toBeNull()
    expect(() => cache.write('credentials', { accessToken: 'x' })).not.toThrow()

    globalThis.localStorage = originalLocalStorage
  })

README.md:188

• If the section above is promoted to a ## heading, these subsections should be ### (not ####) to keep the heading hierarchy consistent.

#### `BackendServerError` and `shouldSkipBackendError()`

[Copilot]

Pull request overview

Copilot reviewed 7 out of 8 changed files in this pull request and generated no new comments.