Issue #3565358: Treat tilde-prefixed maps as composite collections

Custom collections from leaflet_more_maps (those with ~ prefix like ~My-Custom-Map) are now treated as single selectable base layer entries in the Map Bundle form, rather than being expanded into individual layers.

This reduces noise in the form and matches the intended use case: when creating a custom collection with multiple switchable base layers, users want to use the whole collection as one option.

Changes:

• MapBundleForm::getData(): Detect ~ prefix and create single entry
• leaflet_layers.module: Copy all layers when composite layer_id

Closes #3565358