Use email as host login credential; remove separate username field; v2.0.8

- Login form now asks for email + password (no username)
- Admin creates hosts with name + email + password; email is always
  required and welcome email always sends on creation
- Duplicate email check replaces duplicate username check
- Admin table shows Email (login) column only; username field removed
  throughout data model, routes, and UI

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

package.json

@@ -1,7 +1,7 @@
 {
   "name": "siteforge",
   "private": true,
-  "version": "2.0.7",
+  "version": "2.0.8",
   "type": "module",
   "scripts": {
     "dev": "vite",