Support multi-account authentication

[jaxson]

Problem

The CLI authenticates as a single HEY account, but many users have multiple HEY accounts (e.g. personal, custom domain, work). Postings from all accounts appear in box listings, but hey threads and hey reply return 404 for topics belonging to non-authenticated accounts.

Current workaround

Open the app_url in the browser for topics on other accounts. This breaks the CLI workflow.

Proposed solution

Support storing and routing between multiple authenticated accounts, keyed by account_id. When a command targets a specific topic/posting, automatically route to the correct account's credentials based on the account_id field.

Possible UX:

• hey auth login could support adding additional accounts
• hey auth list to show all authenticated accounts
• Commands automatically select the right account based on the posting's account_id
• Fallback: --account flag for explicit selection