Add process-wide Settings model (ticket 008)

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

src/api.rs

@@ -1,5 +1,8 @@
+use std::sync::{Arc, Mutex};
+
 use crate::engine::{EngineHandle, EngineMessage, EngineQuery, EngineResponse};
 use crate::dto::{LogsParams, TimelineParams};
+use crate::settings::Settings;
 use axum::extract::{Query, State};
 use axum::http::StatusCode;
 use axum::response::{IntoResponse, Json};
@@ -8,13 +11,20 @@ use axum::Router;
 use tokio::sync::oneshot;
 use tower_http::services::{ServeDir, ServeFile};
 
+/// Shared, process-wide settings handle. Cheap to clone; lock is only held
+/// briefly since settings changes are rare (unlike the engine's hot path).
+pub type SettingsHandle = Arc<Mutex<Settings>>;
+
 #[derive(Clone)]
 struct ApiState {
     engine: EngineHandle,
+    // TODO(ticket 010/011): wire up settings/CSS/NavMenu endpoints using this.
+    #[allow(dead_code)]
+    settings: SettingsHandle,
 }
 
-pub fn router(engine: EngineHandle, static_dir: &str) -> Router {
-    let state = ApiState { engine };
+pub fn router(engine: EngineHandle, static_dir: &str, settings: SettingsHandle) -> Router {
+    let state = ApiState { engine, settings };
     let index_html = format!("{static_dir}/index.html");
     let static_service = ServeDir::new(static_dir).fallback(ServeFile::new(index_html));