Rename W axis -> A axis everywhere (with migration)

Auxiliary axis is the auxcnc-driven stepper exposed to gplan as A,
not W. Half the stack already used A (gcode, DRO row, soft limits,
homing); the other half (settings tab, macros, internal field
names) still said W which was confusing.

Renames:
  - aux.json fields:       min_w/max_w        -> min_mm/max_mm
  - svelte component:      WAxisSettings      -> AAxisSettings
  - settings tab slug:     #w-axis            -> #a-axis
  - js view module:        w-axis-view.js     -> a-axis-view.js
  - pug template:          w-axis-view.pug    -> a-axis-view.pug
  - macros:                w_down.nc/w_up.nc  -> a_down.nc/a_up.nc
                           'W Down'/'W Up'    -> 'A Down'/'A Up'
  - css class & ids:       .w-axis-settings   -> .a-axis-settings,
                           min_w/max_w form ids match field names
  - internal js identifiers and comments

Migration:
  - AuxAxis._migrate_legacy_fields() promotes min_w/max_w in aux.json
    on every load and persists the upgraded form, so existing
    installs come out clean on first restart.
  - Config._upgrade() renames macro file_name and display name in
    every config.json load, so a stale in-memory copy can't
    reintroduce the W names. Ships with a save right after.

The auxcnc ESP wire protocol verbs (WPOS/HOMED) are unchanged - they
are wire-format identifiers, not user-facing labels.

src/pug/templates/a-axis-view.pug

@@ -0,0 +1,4 @@
+script#a-axis-view-template(type="text/x-template")
+  #a-axis-page
+    h1 A Axis (auxcnc)
+    #a-axis-mount

src/pug/templates/control-view.pug

@@ -249,7 +249,7 @@ script#control-view-template(type="text/x-template")
                   @click=`home('${axis}')`)
                   .fa.fa-home
 
-          // Legacy W axis row - shown only when the auxcnc stepper is
+          // Legacy auxiliary-axis row - shown only when the auxcnc stepper is
           // *not* exposed as a virtual A axis. After v2 the standard
           // A row above renders this axis natively (with full offset
           // + set-position support); this row only appears on legacy

src/pug/templates/settings-shell.pug

@@ -46,7 +46,7 @@ script#settings-shell-view-template(type="text/x-template")
         :index="index", :config="config", :template="template", :state="state")
       io-view(v-if="sub === 'io' && config_ready",
         :index="index", :config="config", :template="template", :state="state")
-      w-axis-view(v-if="sub === 'w-axis' && config_ready",
+      a-axis-view(v-if="sub === 'a-axis' && config_ready",
         :index="index", :config="config", :template="template", :state="state")
       macros-view(v-if="sub === 'macros' && config_ready",
         :index="index", :config="config", :template="template", :state="state")

src/pug/templates/w-axis-view.pug

@@ -1,4 +0,0 @@
-script#w-axis-view-template(type="text/x-template")
-  #w-axis-page
-    h1 W Axis (auxcnc)
-    #w-axis-mount