Introduces src/settings.rs with a single, process-wide Settings struct
(colour scheme, CSS entries shaped for imported-vs-linked, and a
profiles placeholder for ticket 009), JSON-serialized via serde_json to
keep deps minimal. Default settings path is computed with the
directories crate (ProjectDirs::from("", "", "ottyl")) and overridable
via a new --settings-path CLI arg. Settings::load creates and persists
a default file on first run; Settings::save persists changes.
Wires a single Arc<Mutex<Settings>> into main() and threads it through
api::router as an additive ApiState field for tickets 010/011 to build
on.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o
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