feat(platform-client): getAvatar to fetch a user's profile picture

[ABujalance]

Adds PlatformClient.getAvatar(accountId) so apps (CDE, collaboration menus) can show member profile pictures. Returns the raw image Blob from the existing bearer-authed GET /account/:id/avatar route; callers turn it into an object URL. Also adds a responseType: 'blob' option to the base client's request() to support non-JSON responses.

Bearer/context auth only, so it lives on PlatformClient, not the token client.

Usage

const blob = await client.getAvatar(accountId);
const src = URL.createObjectURL(blob); // revoke when done

Rejects when the member has no avatar.

Changes

• getAvatar(accountId): Promise<Blob> on PlatformClient
• responseType: 'json' | 'blob' option on the base client request()
• Unit test + changeset (minor)