Audit and update buttons on Projects::BoardsController#index

What does this MR do?

Updates the following buttons on the Projects::BoardsController#index:

• Add list dropdown to use primary confirm variant.
• Focus mode button to use tertiary default variant.
• Focus mode button only visible at larger breakpoints (see !57139 (merged) description for more details).
• Vertically center chevron in Add list dropdown at small breakpoints.

Screenshots

Before	After

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

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Security

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Related to #273306 (closed)