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/package.json

@@ -10,6 +10,7 @@
     "lint": "tsc --noEmit"
   },
   "dependencies": {
+    "@fontsource-variable/geist": "^5.3.0",
     "clsx": "^2.1.1",
     "dompurify": "^3.4.14",
     "framer-motion": "^11.18.2",