LayerChooser: Support multiple service IDs per entry

[dopenguin]

Description

Currently, each entry in the LayerChooser maps to exactly one (technical) service, because LayerConfiguration in src/core/types/layer.ts references a single id: string.

The goal of this feature is to let two (or more) services appear as a single entry in the LayerChooser and be toggled on/off together. To achieve this, the id parameter should accept string[] in addition to string. When an array is given, the entry is rendered once in the UI (name, legend, availability), while internally the visibility of all referenced OpenLayers layers is controlled in sync.

Agreed behavior for this feature:

• Entry identity (internal key): Use a synthetic key derived from joining the IDs with the separator '|' (which should not occur in a service id, avoiding collision with the comma-separated WMS LAYERS param).
• Zoom availability: A merged entry is considered available if at least one of its layers is available at the current zoom level.
• Legends: If multiple layers of a merged entry provide a legend image, the images are stacked vertically in the legend view.

Affected areas

File	What needs to change
src/core/types/layer.ts	Widen LayerConfiguration.id from string to string | string[]; update TSDoc and document the merged-entry behavior
src/plugins/layerChooser/utils/getBackgroundsAndMasks.ts	Resolve rawLayerList.getLayerWhere({ id }) for each ID in the array; log per missing sub-ID
src/plugins/layerChooser/store.ts	Update activeBackgroundId/activeMaskIds, disabledBackgrounds/disabledMasks, visibleMaskIds, masksSeparatedByType, the watch handlers and setActiveMaskIdsVisibility to match every sub-ID against layer.get('id') while keying entries via the synthetic key
src/plugins/layerChooser/store.ts (updateActiveAndAvailableLayersByZoom, areLayersActive)	Treat a merged entry as available if at least one sub-layer is available at the current zoom
src/plugins/layerChooser/utils/areLayersActive.ts	Support entries with multiple IDs when evaluating zoom/availability
src/plugins/layerChooser/utils/prepareLegends.ts	Collect a legend URL per sub-ID so a merged entry can hold multiple legend images
src/plugins/layerChooser/types.ts	Extend LayerLegend to carry multiple images (e.g. url: string → list of { name, url }) for stacked rendering
src/plugins/layerChooser/components/LayerLegend.ce.vue	Render multiple legend images stacked vertically
src/plugins/layerChooser/components/LayerSelection.ce.vue	Use the synthetic key for :key, v-model values, disabledMasks[...], legend/options button conditions; hide the options button for merged entries
src/core/utils/checkServiceAvailability.ts	Iterate the availability check over all sub-IDs of a merged entry
src/core/utils/map/setupStyling.ts	Verify the register.find((l) => l.id === layer.id) lookup handles merged entries (an array ID resolves to multiple raw layers) or explicitly excludes them

Recommendation on entry identity

Introduce a central helper (e.g. normalizeLayerIds(id: string | string[]): string[]) plus a getLayerKey(id) that joins the IDs with '|'. This key is used wherever the entry must be uniquely referenced in the store (activeMaskIds, activeBackgroundId, disabledMasks, openedLegendId, etc.), while the individual IDs are used for all OpenLayers matching via layer.get('id'). Keeping normalization in one place avoids scattering Array.isArray checks across the codebase.

Out of scope

• options.layers (WMS sub-layer selection) for merged entries: Not supported when id is an array. The options button is hidden for such entries; single-ID entries keep the existing behavior.
• Markers on merged entries: The markers feature (configuration.markers, src/core/utils/map/setupMarkers.ts) only supports single-ID layers. A merged LayerChooser entry (id: string[]) cannot be used as a marker layer; MarkerLayer.id remains a single string.
• GFI / Filter / Attributions on merged entries: Feature queries via combined entries (src/plugins/filter, src/plugins/attributions, GFI) are not part of this issue.
• Mixing layer types in one entry: Combining background and mask or other ids in a single entry is excluded; all sub-IDs share the same type.
• Automatic merging by heuristic: No automatic grouping; merging is only done explicitly via id: string[].