Add guided workflow for assigning miners to a miner controller with no assigned miners

[markoceri]

After creating a miner controller, it is unclear how to assign a miner to it. Currently, the UI simply displays "No miners assigned" (see screenshot below) without providing any actionable guidance or next steps.

Current Behavior

When a miner controller has no miners assigned, the "Assigned Miners" section only shows the text "No miners assigned" with no call-to-action or link to help the user proceed.

Expected Behavior

When a miner controller has no assigned miners, the UI should guide the user with one of the following flows:

1. Available miners exist (miners not assigned to any other miner controller):

   • Display an interface (e.g., a selection list or modal) that allows the user to pick from the list of available (unassigned) miners and assign them to the current miner controller.

2. No miners have been created yet:

   • Display a message inviting the user to create miners first.
   • Provide a direct link/button to navigate to the miner creation page.

Acceptance Criteria

• When a miner controller has no assigned miners and unassigned miners are available, show a UI element (e.g., button or inline prompt) to assign miners from a list of available ones.
• The selection interface should only list miners that are not already assigned to another miner controller.
• When no miners exist at all, display a message prompting the user to create a miner, with a link/button redirecting to the miner creation page.
• The "No miners assigned" empty state should always include a clear call-to-action.