Add optional createdAt property to last used identities

[lmuntaner]

Motivation

We want to add when the identity was created in the identity switcher.

In this PR, I add a new property to the data stored in local storage that is then used in the identity switcher.

Changes

• Added an optional created_at field to the AuthnMethodSessionInfo type in the backend interface and ensured it is populated from the anchor's creation time.
• Updated AuthFlow and RegisterAccessMethodFlow classes to extract, convert (using the new nanosToMillis utility), and pass createdAtMillis when handling identity and authentication method information.
• For legacy identities or flows where created_at is not available, set createdAtMillis to undefined.
• Extended the LastUsedIdentity type and the lastUsedIdentitiesStore to include the optional createdAtMillis field and updated the store's API accordingly.
• Added a new nanosToMillis utility function for converting nanosecond timestamps to milliseconds.

Tests

• Tested locally that the new property is added to the local storage with the right units. See screenshot attached.
• Updated tests for lastUsedIdentitiesStore and lastUsedIdentityStore to include checks for the new createdAtMillis property, ensuring correct behavior and coverage.

[Copilot]

Pull Request Overview

This PR adds an optional createdAtMillis property to the last used identities feature, enabling the identity switcher to display when each identity was created.

Key changes:

• New utility function to extract creation time from identity info
• Updated last used identity store to include optional creation timestamp
• Modified authentication flows to pass creation time when available

Reviewed Changes

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

Show a summary per file

File	Description
src/frontend/src/lib/utils/identityInfo.ts	New utility function to convert identity creation time to milliseconds
src/frontend/src/lib/utils/identityInfo.test.ts	Test coverage for the new utility function
src/frontend/src/lib/stores/last-used-identities.store.ts	Added optional createdAtMillis property to store types
src/frontend/src/lib/stores/last-used-identities.store.test.ts	Updated tests to include the new property
src/frontend/src/lib/flows/registerAccessMethodFlow.svelte.ts	Extract and pass creation time from identity info
src/frontend/src/lib/flows/migrationFlow.svelte.ts	Set undefined for legacy identities without creation time
src/frontend/src/lib/flows/authFlow.svelte.ts	Pass creation time for existing identities or current time for new ones

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[sea-snake]

LGTM