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>

src/components/NuggetsHostPortal.tsx

@@ -12,7 +12,7 @@ export function NuggetsHostPortal() {
   const [hostName, setHostName] = useState('')
   const [seafileConfigured, setSeafileConfigured] = useState(true)
 
-  const [username, setUsername] = useState('')
+  const [email, setEmail] = useState('')
   const [password, setPassword] = useState('')
   const [loginError, setLoginError] = useState('')
   const [loggingIn, setLoggingIn] = useState(false)
@@ -48,7 +48,7 @@ export function NuggetsHostPortal() {
       const r = await fetch('/api/nuggets-upload/login', {
         method: 'POST',
         headers: { 'Content-Type': 'application/json' },
-        body: JSON.stringify({ username, password }),
+        body: JSON.stringify({ email, password }),
       })
       const d = await r.json()
       if (!r.ok) { setLoginError(d.message ?? 'Login failed.'); return }
@@ -63,7 +63,7 @@ export function NuggetsHostPortal() {
     await fetch('/api/nuggets-upload/logout', { method: 'POST' })
     setAuthenticated(false)
     setHostName('')
-    setUsername('')
+    setEmail('')
     setPassword('')
     setSubmitted([])
     resetForm()
@@ -166,9 +166,9 @@ export function NuggetsHostPortal() {
           <p style={s.subText}>Sign in with your host credentials to submit an episode.</p>
           <form onSubmit={handleLogin} style={s.form}>
             <div style={s.field}>
-              <label style={s.label} htmlFor="hp-u">Username</label>
-              <input id="hp-u" style={s.input} type="text" autoComplete="username"
-                value={username} onChange={e => setUsername(e.target.value)} disabled={loggingIn} required />
+              <label style={s.label} htmlFor="hp-u">Email</label>
+              <input id="hp-u" style={s.input} type="email" autoComplete="email"
+                value={email} onChange={e => setEmail(e.target.value)} disabled={loggingIn} required />
             </div>
             <div style={s.field}>
               <label style={s.label} htmlFor="hp-p">Password</label>