Implement isStyle validation instead of always returning true

[tmshv]

Problem

src/store/effects/add-file.ts:8-11 has a stub function that accepts any object as a valid MapLibre style:

function isStyle(value: object): boolean {
    // TODO check the value is real maplibre style
    return true
}

This means any JSON file will be treated as a valid map style, causing confusing runtime errors when the invalid style is applied.

What needs to change

Implement basic validation — at minimum check for required MapLibre style spec fields like version, sources, and layers. The MapLibre style spec defines the expected structure.