Remember the rider, not only the hardware

`RiderConfig` lived in `RideInputs` and nowhere else, and nothing in the
UI ever called `set_rider_config`. So every ride was ridden as the
struct's own default — a 105 kg rider on an 8 kg bike — with no way to
say otherwise short of editing the source. Mass is not a preference: it
sets the speed a given power produces, the ETA that follows from it, the
calorie estimate, and how a 6% ramp feels. FR-7.4 is a Must, and a
command the UI never calls does not satisfy it.

- settings.rs persists rider config, safety limits and display
  preferences to app_data_dir()/settings.json, written atomically and
  read back before the first tick, so no snapshot is ever computed
  against the default. Advisory like known.rs: an unreadable file costs
  the rider their setup, never their ride.
- A stored file is refused *whole* if it fails the same checks the
  commands apply. It may predate a tightened bound or have been edited
  by hand, and a zero mass reaching the engine divides by itself on the
  next tick.
- The commands validate with instructions rather than codes — "CdA must
  be between 0.1 and 1.5 m² — a road position is about 0.32" — because
  this is now a form a rider fills in, not a struct only I ever touched.
- Preferences (FTP, maximum heart rate, units) are Tauri-side, not in
  `RiderConfig`. None of it reaches the physics, and crates/core is the
  frozen contract the engine and the FIT writer share. Zero is a real
  answer for both references and means "no zones", not "unset and
  guessed at".
- SettingsScreen commits on field-exit and reseats every input from what
  Rust returned, so a rejected value can never sit on screen looking
  accepted. Weight, FTP and units are on top; the eight settings with a
  defensible default are folded away.
- Reachable on `,` from any screen, returning to whichever screen opened
  it. Setup swallows the ride controls while it is up — a stray arrow
  key while reading the form must not trim the gradient of a ride
  happening behind it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -15,6 +15,7 @@ pub mod devices;
 pub mod events;
 pub mod heart_rate;
 pub mod known;
+pub mod settings;
 pub mod profile_view;
 pub mod recording;
 pub mod samples;
@@ -98,6 +99,8 @@ pub fn run() {
             commands::set_rider_config,
             commands::safety_limits,
             commands::set_safety_limits,
+            commands::preferences,
+            commands::set_preferences,
             // profiles
             commands::load_profile_from_path,
             commands::load_profile_from_text,
@@ -130,6 +133,20 @@ pub fn run() {
                 Ok(path) => handle.state::<AppState>().lock().devices.attach_store(path),
                 Err(e) => tracing::warn!(error = %e, "remembered devices unavailable"),
             }
+            // FR-7.4: the rider's mass, bike and drag, before the first tick
+            // reads them. Restored ahead of the ride loop starting so no
+            // snapshot is ever computed against the 105 kg default the struct
+            // falls back to.
+            match settings::store_path(&handle) {
+                Ok(path) => {
+                    let state = handle.state::<AppState>();
+                    let mut inner = state.lock();
+                    let inner = &mut *inner;
+                    let (rider, limits) = (&mut inner.inputs.rider, &mut inner.inputs.limits);
+                    inner.settings.attach(path, rider, limits);
+                }
+                Err(e) => tracing::warn!(error = %e, "rider settings unavailable"),
+            }
             // NFR-7: scanning starts immediately, not on a user click.
             handle.state::<AppState>().lock().devices.start_scan();
             state::spawn_ride_loop(handle.clone());