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/manifest.ts

@@ -0,0 +1,23 @@
+import type { MetadataRoute } from "next";
+
+/**
+ * What lets the webmail be installed like an app: its own window, its own
+ * icon, and a splash that matches the ground it opens onto.
+ */
+export default function manifest(): MetadataRoute.Manifest {
+  return {
+    name: "LambdaMail",
+    short_name: "LambdaMail",
+    description: "Your own mail server, with a webmail that is a pleasure to use.",
+    start_url: "/user/mail/inbox",
+    scope: "/",
+    display: "standalone",
+    background_color: "#0F1218",
+    theme_color: "#0F1218",
+    icons: [
+      { src: "/icons/icon-192.png", sizes: "192x192", type: "image/png" },
+      { src: "/icons/icon-512.png", sizes: "512x512", type: "image/png" },
+      { src: "/icons/maskable-512.png", sizes: "512x512", type: "image/png", purpose: "maskable" },
+    ],
+  };
+}