Fix Wind Rose not displayed after new map generation

[Copilot]

Description

The Wind Rose (compass rose) disappears after generating a new map because undraw() removes all <use> SVG elements from the viewbox — including the compass rose's <use xlink:href="#defs-compass-rose"> child. drawLayers() then restores visibility of the #compass group but never re-appends the deleted <use> element, leaving it empty.

Changes in public/modules/ui/layers.js:

• drawLayers() — guard-check before restoring compass visibility; re-append <use> if missing:

  if (layerIsOn("toggleCompass")) {
    if (!compass.select("use").size()) compass.append("use").attr("xlink:href", "#defs-compass-rose");
    compass.style("display", "block");
  }

• toggleCompass() — same guard added when the user manually re-enables the layer after a regeneration cycle, preventing the same blank-compass scenario on explicit toggle.

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