Refactor toolbar slider mapping into the UI model

[devmobasa]

Summary

Moves toolbar slider value conversion out of the Wayland backend and into the toolbar UI model, so hit handling and rendering share explicit model-level helpers instead of duplicating geometry assumptions.

What changed

• Added toolbar model helpers for slider specs, normalized value conversion, pointer hit-rect mapping, visual knob mapping, and delay slider values.
• Updated Wayland hit handling to delegate slider press/drag conversion through the model.
• Updated side palette rendering to use the visual knob mapping helper.
• Kept pointer mapping and visual knob travel separate so hit behavior and rendering geometry can evolve independently.
• Added guard tests for slider mapping, clamping, click-only focus traversal, focused activation, and delay helper behavior.