feat(webmail): the new interface - logo, dark and light themes, command menu

A mark (a lambda on an ultramarine tile with the flap of an envelope), the
favicon, home-screen icons and an install manifest. Every colour is a variable
defined once per theme, and the Tailwind scales the screens already use read
them, so dark (the default) and light are a change of values and not of
classes. Geist is served by the app itself. Ctrl+K opens a command menu for
folders, actions, accounts and appearance, and typing something that matches no
command searches the mail; c writes a message and / goes to the search field.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

services/webmail/src/app/(auth)/admin/step-up/page.tsx

@@ -96,7 +96,7 @@ export default function AdminStepUpPage() {
     <div className="flex min-h-screen items-center justify-center bg-dark-bg px-4">
       <div className="w-full max-w-[400px] rounded-2xl bg-dark-panel p-7 shadow-edge">
         <div className="mb-6 flex items-center gap-3">
-          <div className="flex h-10 w-10 flex-none items-center justify-center rounded-xl bg-dark-card text-indigo-500 shadow-[inset_0_0_0_1px_#9184d9]">
+          <div className="flex h-10 w-10 flex-none items-center justify-center rounded-xl bg-dark-card text-indigo-500 shadow-[inset_0_0_0_1px_rgb(var(--a-500))]">
             <Sliders className="h-5 w-5" />
           </div>
           <div className="min-w-0">