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-lock.json

@@ -8,6 +8,7 @@
       "name": "@lambdamail/webmail",
       "version": "0.1.0",
       "dependencies": {
+        "@fontsource-variable/geist": "^5.3.0",
         "clsx": "^2.1.1",
         "dompurify": "^3.4.14",
         "framer-motion": "^11.18.2",
@@ -718,6 +719,15 @@
         }
       }
     },
+    "node_modules/@fontsource-variable/geist": {
+      "version": "5.3.0",
+      "resolved": "https://registry.npmjs.org/@fontsource-variable/geist/-/geist-5.3.0.tgz",
+      "integrity": "sha512-j0m+vLQuG5XAYoHtGCVu0spvlGreR3EzpECUVzkFmI1mTVnAO38l/NEPDCFgZ177JxzYJCLSmTQibIiYPilGrA==",
+      "license": "OFL-1.1",
+      "funding": {
+        "url": "https://github.com/sponsors/ayuhito"
+      }
+    },
     "node_modules/@img/colour": {
       "version": "1.1.0",
       "resolved": "https://registry.npmjs.org/@img/colour/-/colour-1.1.0.tgz",