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/main.rs

@@ -5,12 +5,18 @@ mod engine;
 mod index_key;
 mod ingest;
 mod metric_primitive;
+mod settings;
 mod timestamp;
 
+use std::path::PathBuf;
+use std::sync::{Arc, Mutex};
+
 use clap::Parser;
 use tokio::net::TcpListener;
 use tokio::sync::mpsc;
 
+use settings::Settings;
+
 /// Observability dashboard from terminal output.
 #[derive(Parser, Debug)]
 struct Args {
@@ -21,17 +27,28 @@ struct Args {
     /// Directory containing the built webapp (index.html + assets).
     #[arg(long, default_value = "webapp/dist")]
     static_dir: String,
+
+    /// Path to the settings file. Defaults to a per-OS config location
+    /// (e.g. `~/.config/ottyl/settings.json` on Linux).
+    #[arg(long)]
+    settings_path: Option<PathBuf>,
 }
 
 #[tokio::main]
 async fn main() {
     let args = Args::parse();
 
+    let settings_path = args.settings_path.clone().unwrap_or_else(|| {
+        settings::default_settings_path()
+            .expect("could not determine a default settings path; pass --settings-path")
+    });
+    let settings = Arc::new(Mutex::new(Settings::load(&settings_path)));
+
     let (engine_tx, engine_rx) = mpsc::channel(1024);
     tokio::spawn(engine::run(engine_rx));
     tokio::spawn(ingest::run(engine_tx.clone()));
 
-    let app = api::router(engine_tx, &args.static_dir);
+    let app = api::router(engine_tx, &args.static_dir, settings);
     let listener = TcpListener::bind(("0.0.0.0", args.port))
         .await
         .expect("failed to bind listener");
@@ -45,3 +62,23 @@ async fn main() {
         .await
         .expect("server error");
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn settings_path_defaults_to_none() {
+        let args = Args::parse_from(["ottyl"]);
+        assert_eq!(args.settings_path, None);
+    }
+
+    #[test]
+    fn settings_path_flag_overrides_default() {
+        let args = Args::parse_from(["ottyl", "--settings-path", "/tmp/custom-settings.json"]);
+        assert_eq!(
+            args.settings_path,
+            Some(PathBuf::from("/tmp/custom-settings.json"))
+        );
+    }
+}