refactor(core): split the config tests by what they exercise

config/tests.rs was 2266 lines: 93 tests over eleven areas of Config, one after another. Each area is its own module now — files, keyboard, lighting, device_settings, identity, per_app, app_settings, schema, gestures, migrations, links — and tests.rs keeps the imports and the one fixture they all share (write_and_read). Every child starts with `use super::*`; the one include_str! path gained a level for the deeper directory.

Pure move: no test body changed; 93 tests before and after.