feat: Admin-Benutzerverwaltung + Dashboard-Layout-Fixes

Backend:
- AdminEndpoint.cs: 5 Endpunkte (List, Create, Update, Reset PW, Delete) - Admin-only
- DatabaseService: CreateUser(), UpdateUserAdmin(), DeleteUser() (kaskadiert Tokens + Shares)
- Route in Program.cs registriert

Frontend:
- admin-page.js: User-Liste mit Edit/Delete, Inline-Formular Create/Edit
- api.js: 5 Admin-API-Funktionen
- index.html: Template tpl-admin, Script-Tag, Dashboard-Link 'Benutzerverwaltung'
- dashboard.js: Admin-Link nur für Admins sichtbar
- form.css: Styles für admin-user-list, admin-user-row, admin-checkbox-row
- dashboard.css: Header-Overflow-Fix, Agents vor Cronjobs
- index.html: Template-Reihenfolge Agents vor Cronjobs

TODO: User-Management als erledigt markiert

Services/DatabaseService.cs

@@ -912,6 +912,34 @@ public class DatabaseService
             new { userId, settings, now = DateTime.UtcNow.ToString("o") });
     }
 
+    public async Task<UserRecord> CreateUser(string username, string passwordHash, string displayName, bool isAdmin)
+    {
+        using var conn = CreateConnection();
+        var now = DateTime.UtcNow.ToString("o");
+        var userId = Guid.NewGuid().ToString("N");
+        await conn.ExecuteAsync(@"
+            INSERT INTO users (user_id, username, password_hash, display_name, is_admin, created_at, updated_at)
+            VALUES (@userId, @username, @passwordHash, @displayName, @isAdmin, @now, @now)",
+            new { userId, username, passwordHash, displayName, isAdmin, now });
+        return await GetUser(userId);
+    }
+
+    public async Task UpdateUserAdmin(string userId, string username, string displayName, bool isAdmin)
+    {
+        using var conn = CreateConnection();
+        await conn.ExecuteAsync(
+            "UPDATE users SET username = @username, display_name = @displayName, is_admin = @isAdmin, updated_at = @now WHERE user_id = @userId",
+            new { userId, username, displayName, isAdmin, now = DateTime.UtcNow.ToString("o") });
+    }
+
+    public async Task DeleteUser(string userId)
+    {
+        using var conn = CreateConnection();
+        await conn.ExecuteAsync("DELETE FROM tokens WHERE user_id = @userId", new { userId });
+        await conn.ExecuteAsync("DELETE FROM shared_messages WHERE user_id = @userId", new { userId });
+        await conn.ExecuteAsync("DELETE FROM users WHERE user_id = @userId", new { userId });
+    }
+
     // ── Telegram (Session-basiert) ──
 
     public async Task<SessionRecord> GetSessionByTelegramChatId(long chatId)