FilterableMap uses light map tiles in dark mode

## Bug When the app is in dark mode, the FilterableMap component still renders light-themed Mapbox/MapTiler tiles. The surrounding UI is dark but the map is bright white/light, creating a harsh visual contrast. ## Expected The map should use dark-themed tiles when the app is in dark mode. Mapbox and MapTiler both offer dark map styles (e.g., `mapbox://styles/mapbox/dark-v11` or MapTiler's `dataviz-dark`). ## Visible on - Discover page (logged-out homepage with map) - Discover page (logged-in) - Organization profile page (map section) - Ring detail page (map section) ## Fix In the MapView component, detect the current theme and switch the map style URL: - Light mode: current light tiles - Dark mode: dark variant tiles The theme is available via the existing theme context/cookie (`gathrings_theme`).