Rename user-facing 'W axis' references to 'A axis' / 'auxiliary axis'

The auxcnc-driven stepper has been integrated into gplan as a
virtual A axis since the option-b migration. User-facing labels
that still said 'W axis' are now confusing because the user types
G-code with A, sees 'A' in the DRO, but config tabs and tooltips
still talked about W.

Cleaned up:
  - Settings tab label W Axis -> A Axis (route slug stays #w-axis
    for back-compat with bookmarks)
  - WAxisSettings.svelte tooltips and tip text say 'auxiliary' or
    just describe the field generically
  - control-view.pug DRO row tooltips, comments, labels
  - control-view.js console.error messages and route-comments
  - axis-vars.js _compute_aux_axis tooltips
  - AuxAxis.py boot-banner message

Internal identifiers (filenames WAxisSettings.svelte, route slug
#w-axis, aux.json fields min_w/max_w, internal comments referring
to the historical W-as-aux design) are kept where they are tied to
on-disk state or wire formats - those rename moves are not
worthwhile and would force users to migrate their aux.json files.

src/pug/templates/control-view.pug

@@ -219,7 +219,7 @@ script#control-view-template(type="text/x-template")
               // Master Home All. Each row's Actions cell has a per-axis
               // home button; this header-level button homes every
               // enabled axis (legacy Onefinity behavior). Auto-includes
-              // the W axis when it is enabled.
+              // the auxiliary A axis when it is enabled.
               button.icon-btn(:disabled="!is_idle",
                 title="Home all axes.", @click="home_all()")
                 .fa.fa-house-chimney
@@ -252,8 +252,8 @@ script#control-view-template(type="text/x-template")
           // Legacy W 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); the W row stays for backwards
-          // compatibility with installs that haven't migrated.
+          // + set-position support); this row only appears on legacy
+          // installs that haven't migrated yet.
           .dro-row(:class="w.klass + ' ' + w.tklass",
             v-if="w.enabled && !a.enabled",
             :title="w.title")
@@ -268,7 +268,7 @@ script#control-view-template(type="text/x-template")
                 .fa.fa-location-dot
               button.icon-btn(:class="w.homed ? 'state-green' : 'state-amber'",
                 :disabled="!w.enabled",
-                title="Home W axis.", @click="aux_home()")
+                title="Home auxiliary axis.", @click="aux_home()")
                 .fa.fa-home
 
         // ----- Status strip -----