Function apply_settings_key 

fn apply_settings_key(
    current: &Settings,
    key: &str,
    value: Value,
) -> Result<Settings, String>

Merge a single key/value override into current and return the resulting Settings, ready to store.

Pure (no locks, no runtime), so the JSON round-trip + clamp is unit-testable without driving the production AppHandle<Wry> IPC path. Serialises current to JSON, validates the key exists, swaps in value, deserialises back, and runs clamp() — the same normalisation the GUI update_settings command applies. Without it the IPC/CLI path could persist out-of-range values the GUI forbids (e.g. a 0s interval that makes a break fire every tick) and skip custom_css/fixed-time sanitisation. clamp() rebuilds the #[serde(skip)] derived cache as its final step, so the wholesale-deserialise’s empty cache is repopulated. Errors are the user-facing strings the IPC handler returns verbatim.