Padlock-on-a-shield mark, dark and light themes, history note

A new mark: a padlock on a shield with the W as its keyhole, in the
console's teal on a dark shield. It replaces the placeholder W tile in the
sidebar, on the sign-in and setup pages, and as the favicon, with app icons
and a web manifest. The SVG sources, a single-colour variant, a wordmark
and a social preview image live in docs/brand.

The console now has a theme choice: dark by default, light, or follow the
system, switched from the sidebar or the Account page and remembered per
browser. The light palette is a first-class theme rather than a media
query, so both are laid out the same way.

The README opens with the mark and gains a History section about the 2025
WGX installer this replaces.

docs/brand/wgx-mark-mono.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 128 128" width="128" height="128" role="img" aria-label="WGX">
+  <!-- single-colour version: inherits currentColor, for print, status bars and anywhere one ink -->
+  <g fill="none" stroke="currentColor" stroke-linejoin="round" stroke-linecap="round">
+    <path d="M64 8 L112 23.5 V60 C112 88.5 92 109.5 64 119.5 C36 109.5 16 88.5 16 60 V23.5 Z" stroke-width="7"/>
+    <path d="M48 64 V52 a16 16 0 0 1 32 0 V64" stroke-width="7.5"/>
+  </g>
+  <path fill-rule="evenodd" fill="currentColor" d="
+    M47 60 h34 a9 9 0 0 1 9 9 v20 a9 9 0 0 1 -9 9 h-34 a9 9 0 0 1 -9 -9 v-20 a9 9 0 0 1 9 -9 z
+    M52 70 l6 17.5 l6 -8.5 l6 8.5 l6 -17.5 l-4.6 -1.6 l-3.6 10.5 l-3.8 -5.4 l-3.8 5.4 l-3.6 -10.5 z"/>
+</svg>