Element Desktop (Electron) loads the location-sharing map style from a
`vector://vector` origin, so fetching map_style.json from the Element
domain is a cross-origin request. Without an Access-Control-Allow-Origin
header, the request is blocked and maps fail to load with "This homeserver
is not configured correctly to display maps".

Add a dedicated Traefik router for map_style.json that attaches an
Access-Control-Allow-Origin header (configurable via
matrix_client_element_location_sharing_map_style_access_control_allow_origin,
defaulting to `*`). It is only defined when location sharing is enabled.

Fixes #2291

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>