Refactored user/volunteer/participant identity.

frontend/src/pages/Login.svelte

@@ -4,7 +4,7 @@
 
   let { onlogin } = $props()
 
-  let username = $state('')
+  let email = $state('')
   let password = $state('')
   let error = $state('')
   let loading = $state(false)
@@ -14,7 +14,7 @@
     error = ''
     loading = true
     try {
-      const { token, user } = await api.login(username, password)
+      const { token, user } = await api.login(email, password)
       await saveSession(token, user)
       onlogin({ token, user })
     } catch (err) {
@@ -34,8 +34,8 @@
     {/if}
     <form onsubmit={submit}>
       <div class="form-group">
-        <label for="username">Username</label>
-        <input id="username" bind:value={username} autocomplete="username" required />
+        <label for="email">Email</label>
+        <input id="email" type="email" bind:value={email} autocomplete="email" required />
       </div>
       <div class="form-group">
         <label for="password">Password</label>