Web UI: no account management/switcher for multi-account providers (CLI-only feature)

Summary

Recent opencode2 CLI builds support multiple accounts per provider (e.g., opencode2 auth list shows several "OpenCode Go" accounts: default, 2nd, …), but the Web UI offers no way to see which account is active or to manage/switch accounts — the feature is CLI-only.

Environment

• opencode version: 0.0.0-beta-17963+ (observed on beta-18314)
• Interface: Web UI (background service), CLI opencode2 auth
• Provider: OpenCode Go (also OpenCode)

Reproduction / Observation

1. CLI: log in to the same provider multiple times — opencode2 auth list shows multiple stored accounts:

   OpenCode     default                     stored
   OpenCode Go  2nd                         stored
   OpenCode Go  default                     stored
   

2. Open the Web UI (home or settings) — there is no account list, no account switcher, and no indication of which account is currently in use.
3. Requests in the Web UI always use the (default) account with no way to switch.

Expected Behavior

Mirror the CLI's account concept in the Web UI:

• Show which account(s) are configured and which one the session/model provider is using
• Allow adding/logging in, switching, and logging out accounts (at least for OpenCode / OpenCode Go console accounts)

Actual Behavior

• Multi-account is configured and visible only via opencode2 auth list (CLI)
• The Web UI has no account/credential management surface (checked the app source: only i18n strings mention accounts; no account UI component exists)

Suggested fix direction

• Add an account section to the Web UI settings (list configured accounts per provider, mark the active one, log out/switch)
• Ensure the active account choice is surfaced per session/model (e.g., provider label) so users can tell which account is billed

Notes

• Searched issues for an existing request covering "multi-account web UI" — none found (please re-check before triage)