LayerChooser: Support multiple service IDs per entry

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

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[].

Start with src/core/types/layer.ts and trace LayerChooser state through src/plugins/layerChooser/store.ts, using the listed utility and component files as the implementation map. Define the shared identity and ID normalization behavior, then verify merged entries render once, synchronize all referenced layers, report availability correctly, stack legends, and preserve the stated out-of-scope behavior.