refactor(components): unify tablist accessibility and keyboard logic

After we close #251 we should plan a refactor to avoid having duplicate logic in Tabs and ContentSwitch.

Description

Currently, the Tabs and ContentSwitch components share the same underlying WAI-ARIA tablist and tab semantics. However, the logic for keyboard navigation (Arrow keys, Home, End, Space, Enter) and roving tabindex is implemented independently in each component.
Note that the correct implementation is the one found in ContentSwitch.

This duplication increases the maintenance surface and risks inconsistent accessibility behaviors across the library. This refactor aims to extract the common logic into a single source of truth.

Goals

• Ensure identical keyboard navigation and focus management for all tab-based components.
• Reduce code duplication in Tabs and ContentSwitch.
• Centralize the management of ARIA attributes (e.g., aria-selected, tabindex).

Potential Implementation

The shared logic could be extracted into a reusable module. Possibilities include:

• A headless Svelte Action to manage DOM events and focus.
• A centralized utility or custom store to handle the state and keyboard mapping.

After #251 is closed, compare the keyboard navigation, roving tabindex, and ARIA logic in the Tabs and ContentSwitch components, using ContentSwitch as the reference. Evaluate whether a shared Svelte Action, utility, or custom store best fits the existing components. Done means both components share one implementation and have identical keyboard, focus, and ARIA behavior.