feat(account): remembered accounts switcher and avatar cache bust
- Add HMAC-signed atmo_accounts cookie and rememberedAccounts on session state
- OAuth callback appends signed-in identity; add /oauth/switch, /forget, /add-account
- AccountMenu: switch between remembered accounts, forget, add account; DID-keyed avatar
- buildAccountMenuProps: /api/me/avatar?v=<did> so browser cache does not mix users
- Wire Nav account props across explore and admin pages; i18n + styles
Made-with: Cursor