Chatbot: add variant to reveal message actions on hover/focus

[nicolethoen]

Based on the results of the Compass/unified theme dev/design sync on Jan 14:

Context

To support an implementation with less visual clutter, we need a design variant where message actions (copy, edit, delete) can be hidden by default and revealed on hover/focus. This is an opt-in configuration.

Design Requirements

• Interaction States:
  • Default (Hidden): Actions are not visible, but their container maintains its dimensions to prevent layout shifting.
  • Active (Hover/Focus): Actions appear within the reserved space.
• Hover Target: The trigger area must encompass the entire message bubble and the action container area to prevent the actions from "flickering" when the mouse moves between the message and the buttons.
• Visual Transitions: Define the entry/exit animation (e.g., a simple 200ms opacity fade).

Acceptance Criteria

• Spec defines the layout for the "Hidden" state (preserving empty space).
• Spec defines the "Visible" state (hover/focus).
• Visual confirmation that message text and surrounding components do not shift when actions appear.