v0.4.0

Closes the first round of backlog issues (#1–#4).

• Responsive image options (#1): widths, formats (incl. AVIF),
  quality (int, per-format map, or null to fall through to Kirby's
  thumbs config), sizes, and fallback are now plugin options — no
  more snippet fork to change a number.
• Re-init API (#2): window.kirbyImageCompare.init(root) initializes
  dynamically inserted blocks (htmx/Turbo swaps, infinite scroll);
  idempotent per stage.
• ARIA slider (#3): the handle is a proper WAI-ARIA slider
  (role="slider", aria-valuemin/max/now) with full keyboard support —
  arrow keys (configurable step option, default 2 %), PageUp/PageDown
  (10 %), Home/End.
• Single source for the visuals (#4): the Panel preview now bundles
  the frontend stylesheet (kirbyup import) and reuses its classes —
  divider/grip styles exist exactly once; the preview divider moves via
  the same custom property as the frontend.