feat: add generic mail server icon

Custom envelope icon (slate #64748B) triggered by: mail, smtp, imap,
postfix, dovecot, mailcow, mailu, roundcube. Uses the same {title, hex,
path} format as simple-icons so no consumer changes are needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/brandIcons.js

@@ -20,6 +20,13 @@ import {
   siKde,
 } from 'simple-icons'
 
+// Icônes personnalisées (absentes de simple-icons) — même format { title, hex, path }.
+const ICON_MAIL = {
+  title: 'Mail',
+  hex: '64748B',
+  path: 'M20 4H4c-1.1 0-2 .9-2 2v12c0 1.1.9 2 2 2h16c1.1 0 2-.9 2-2V6c0-1.1-.9-2-2-2zm0 4l-8 5-8-5V6l8 5 8-5v2z',
+}
+
 // Ordre : du plus spécifique au plus générique pour éviter les faux positifs.
 const BRANDS = [
   // Hyperviseurs / virtualisation
@@ -59,6 +66,9 @@ const BRANDS = [
   { kw: ['keepass', 'keepassxc'],                         icon: siKeepassxc },
   { kw: ['hashicorp vault', 'hashicorp'],                 icon: siVault },
 
+  // Messagerie
+  { kw: ['mail', 'smtp', 'imap', 'postfix', 'dovecot', 'mailcow', 'mailu', 'roundcube'], icon: ICON_MAIL },
+
   // Bases de données
   { kw: ['mariadb', 'maria db'],                          icon: siMariadb },