Subtask: Remove 'Buy Ruuvi sensors' link from empty dashboard

[ojousima]

Summary

Remove the "Buy Ruuvi sensors" link from the dashboard empty-state card (signed-out/no-sensors view).

Parent

Subtask of #1532

Scope

• Remove the underlined "Buy Ruuvi sensors" link shown below the "Add a sensor" button in the empty dashboard state.
• Keep Sign in / Add a sensor actions intact.

Acceptance criteria

• The empty dashboard state no longer displays the "Buy Ruuvi sensors" link.
• No regressions to sign-in or add-sensor actions in this view.