Refactor volume knob interaction into explicit controller and normalize mouse behavior

[Copilot]

The volume knob interaction was hard to reason about and felt inconsistent with mouse input. This change makes knob behavior explicit and predictable: click-to-set, drag-to-adjust, release-to-stop.

• Interaction model (OOP)

  • Introduced VolumeKnobController (volume_knob.py) to own knob semantics:
    • hit-testing (contains, configurable hit_padding)
    • drag lifecycle (start_drag, drag, stop_drag)
    • angle/value mapping and clamping
    • sprite rotation mapping (sprite_rotation_degrees)
  • Moved knob math/state transitions out of the main event loop into named methods.

• Main loop integration

  • Replaced ad-hoc knob handling in main.py with controller calls.
  • Volume now updates immediately on knob press (not only after motion), and continuously during drag.
  • Centralized mixer update via _apply_volume_from_knob() to avoid duplicated apply logic.
  • Drag updates now apply only when knob value actually changes.

• UI/UX consistency

  • Preserved existing layout and element positions (no new overlapping UI).
  • Slightly expanded effective knob hit area for easier grabbing without changing rendered geometry.

• Docs + focused coverage

  • Updated README usage text from drag-only to click-or-drag behavior.
  • Added focused unit tests for controller behavior (tests/test_volume_knob.py): hit area, immediate click update, drag gating, and rotation mapping.

# main.py (event handling)
if event.type == MOUSEBUTTONDOWN and event.button == 1:
    if volume_knob.start_drag(mouse_pos):
        _apply_volume_from_knob()

elif event.type == MOUSEMOTION:
    if volume_knob.drag(mouse_pos):
        _apply_volume_from_knob()

elif event.type == MOUSEBUTTONUP and event.button == 1:
    volume_knob.stop_drag()