v0.2.3

Adds a curtain shutter effect as capture feedback, plus clearer icons.

• A successful snapshot from the toolbar plays a brief shutter animation over the map —
  two translucent blades close to the centre and reopen (the map stays faintly visible) —
  visual confirmation that doubles as the "copied" feedback for the otherwise-silent
  clipboard delivery. Opt out with snapshot: { shutter: false } (default true).
• Honours prefers-reduced-motion (degrades to a single quick dim); the overlay is
  pointer-events:none and self-removes. The flash plays only on success.
• The snapshot button icon is a camera; the two deliveries differ only by the lens —
  filled for download (SNAPSHOT_ICON_SVG), an empty ring for clipboard
  (SNAPSHOT_CLIPBOARD_ICON) — and the hover preview swaps between them.
• New export shutterFlash(container, { durationMs? }) for manual use.