Remember dashboard users for 30 days after login

## Problem

The dashboard UI currently appears to require users to log in again instead of remembering the authenticated browser session for a longer period. This is inconvenient for normal dashboard usage where a trusted browser should stay signed in across restarts and short breaks.

## Desired behavior

After a successful dashboard login, remember the user in the browser for 30 days so they do not need to re-enter credentials/API key on every visit.

## Scope

- Dashboard UI service
- Browser-side persistence/cache for the authenticated session or login token
- Any matching API/session behavior needed to support the 30-day lifetime safely

## Acceptance criteria

- A successful login persists enough auth state for the dashboard to restore the user for up to 30 days.
- Persistence survives page refresh and browser restart.
- Expired or invalid credentials are detected and the user is sent back to login.
- Logout clears the persisted browser state immediately.
- The implementation avoids storing raw secrets unnecessarily; prefer a scoped/session token if available.
- Add or update tests for login restore, expiration, and logout clearing behavior.

## Notes

Use browser storage/cookies deliberately. If the dashboard currently stores an API key directly, consider whether a server-issued session token or explicit expiry metadata is safer for this 30-day remember-me behavior.