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.

web/src/pages/Setup.tsx

@@ -2,6 +2,7 @@ import { useState, type FormEvent } from "react";
 import { api } from "../api";
 import { errorMessage, useAuth } from "../state";
 import { Field } from "../components/ui";
+import { Mark } from "../components/Mark";
 
 export function Setup() {
   const { refresh } = useAuth();
@@ -35,10 +36,8 @@ export function Setup() {
       <form className="card" onSubmit={submit}>
         <div className="card-body">
           <div className="brand">
-            <div className="brand-mark" aria-hidden="true">
-              <svg width="18" height="18" viewBox="0 0 32 32">
-                <path d="M7 10l4 12 5-9 5 9 4-12" fill="none" stroke="currentColor" strokeWidth="3.5" strokeLinecap="round" strokeLinejoin="round" />
-              </svg>
+            <div className="brand-mark">
+              <Mark size={44} />
             </div>
             <div className="brand-name">WGX</div>
           </div>