feat: User-Management mit Token-basierter Authentifizierung

Backend:
- UserRecord Model + Users-Tabelle in DB
- AuthService: SHA256-Passwort-Hashing, Token-Verwaltung (ConcurrentDictionary)
- AuthMiddleware: Schützt alle /api/-Routes (außer /api/auth/login)
- AuthEndpoint: Login, Logout, Me
- Default-User admin/admin wird automatisch erstellt

Frontend:
- Login-Page mit Formular (Enter-Key-Support)
- Globaler Fetch-Interceptor: 401 → automatisch Login-Redirect
- Auth-Check beim App-Start und bei Hash-Änderungen
- Logout-Link im Dashboard-Header
- Login-CSS: Zentrierte Login-Box

wwwroot/index.html

@@ -11,6 +11,7 @@
   <link rel="stylesheet" href="css/base.css">
   <link rel="stylesheet" href="css/main.css">
 
+  <link rel="stylesheet" href="css/login.css">
   <link rel="stylesheet" href="css/form.css">
   <link rel="stylesheet" href="css/dashboard.css">
   <link rel="stylesheet" href="css/chat.css">
@@ -28,12 +29,29 @@
 
 <!-- Templates -->
 
+<template id="tpl-login">
+  <div class="page login-page">
+    <div class="login-box">
+      <h1 class="login-title">// gott</h1>
+      <div class="login-form">
+        <input type="text" id="loginUsername" placeholder="Benutzername" autocomplete="username" autofocus>
+        <input type="password" id="loginPassword" placeholder="Passwort" autocomplete="current-password">
+        <div class="login-error" id="loginError" hidden></div>
+        <button id="loginBtn">Anmelden</button>
+      </div>
+    </div>
+  </div>
+</template>
+
 <template id="tpl-dashboard">
   <div class="page dashboard">
     <header>
       <button class="back-btn icon-btn" disabled></button>
       <h1 class="dashboard-title">// gott</h1>
-      <a id="dashboardSettingsLink" class="dashboard-link">Einstellungen</a>
+      <div class="dashboard-links">
+        <a id="dashboardSettingsLink" class="dashboard-link">Einstellungen</a>
+        <a id="dashboardLogoutLink" class="dashboard-link">Logout</a>
+      </div>
     </header>
     <div class="page-content">
       <section>
@@ -287,6 +305,7 @@
 <script src="js/pages/agent-page.js"></script>
 <script src="js/pages/session-page.js"></script>
 <script src="js/pages/chat-page.js"></script>
+<script src="js/pages/login-page.js"></script>
 <script src="js/pages/settings-page.js"></script>
 <script src="js/app.js"></script>
 </body>