Docs: README + W axis docs cover macOS build/flash and new UI

- README.md (was a one-liner): describe the layout, the macOS quick
  path including the esbuild platform-pin gotcha, and how to flash
  with curl or 'make update'.
- docs/AUX_W_AXIS.md: document the new Control jog row layout, the
  Settings 'W Axis (auxcnc)' section, and list the additional UI
  files touched by this fork.

docs/AUX_W_AXIS.md

@@ -100,7 +100,30 @@ persisted there in NVS.
 Steps-mode jog ignores soft limits (use it to inch the axis to the
 limit switch when the axis isn't homed yet).
 
-## State surface (UI)
+## UI
+
+**Control view**
+
+- A jog row appears under the XYZ jog grid when `aux_enabled` is true,
+  with three buttons: `W-`, `W+`, and a wide `Home W`. There is
+  intentionally no separate "set zero" or "W origin" button - homing
+  lands the axis at `home_position_mm` (0 by default), so home and
+  zero are the same point.
+- The DRO table shows a W axis row with position, status (OFFLINE /
+  UNHOMED / HOMED), and a single Home button in the actions column
+  (the cog and map-marker columns are placeholders for layout).
+
+**Settings view**
+
+A "W Axis (auxcnc)" section exposes every aux.json field except
+`enabled` (which stays read-only - flipping the W axis on/off requires
+editingaux.json on the controller, so a fresh install can't surprise
+the user with hardware that isn't there). Saving PUTs the merged
+config to `/api/aux/config/save`, which writes aux.json and pushes
+`HOMECFG` to the ESP. A status line shows whether the axis is
+disabled / offline / connected-unhomed / homed at `<pos> mm`.
+
+## State surface
 
 These are pushed via `state.set` and visible in the websocket stream:
 
@@ -139,6 +162,11 @@ These are pushed via `state.set` and visible in the websocket stream:
 - `src/py/bbctrl/FileHandler.py`: rewrite uploads in place
 - `src/py/bbctrl/Web.py`: REST endpoints
 - `src/py/bbctrl/__init__.py`: export AuxAxis
+- `src/pug/templates/control-view.pug`: W jog row + DRO row
+- `src/js/control-view.js`: aux_home / aux_jog / aux_jog_incr handlers
+- `src/js/axis-vars.js`: `_compute_aux_axis` for W state
+- `src/svelte-components/src/components/WAxisSettings.svelte`: settings panel
+- `src/svelte-components/src/components/SettingsView.svelte`: hosts WAxisSettings
 - `auxcnc/src/main.cpp`: new commands HOME, HOMECFG, WPOS, HOMED?,
   LIMIT?, ABORT-able STEPS with limit-aware abort, trapezoidal ramps,
   NVS-persisted config, `[boot]` banner, deterministic reply tokens