Add user management, change log and customer archive

- Multi-user auth with username/password (replaces single password)
- Default admin account created automatically (admin/cuitunet2024)
- User CRUD with admin/user roles (only admin can manage users)
- All customer changes logged with timestamp, user and details
- Customer deletion now archives instead of permanently removing
- Archive view with restore and permanent delete options
- Tab navigation: Asiakkaat, Arkisto, Muutosloki, Käyttäjät
- Protect users.json, changelog.json and archive.json in .gitignore

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

style.css

@@ -120,6 +120,13 @@ header {
     align-items: center;
 }
 
+.user-info {
+    font-size: 0.85rem;
+    opacity: 0.8;
+    padding-right: 0.5rem;
+    border-right: 1px solid rgba(255,255,255,0.2);
+}
+
 /* Main container */
 .main-container {
     max-width: 1400px;
@@ -905,3 +912,107 @@ span.empty {
         text-align: center;
     }
 }
+
+/* Tab bar */
+.tab-bar {
+    display: flex;
+    gap: 0;
+    background: #fff;
+    border-bottom: 2px solid #e5e7eb;
+    padding: 0 2rem;
+    position: sticky;
+    top: 56px;
+    z-index: 99;
+}
+
+.tab {
+    padding: 0.75rem 1.25rem;
+    background: none;
+    border: none;
+    border-bottom: 3px solid transparent;
+    font-size: 0.88rem;
+    font-weight: 600;
+    color: #888;
+    cursor: pointer;
+    transition: all 0.2s;
+    margin-bottom: -2px;
+}
+
+.tab:hover {
+    color: #0f3460;
+}
+
+.tab.active {
+    color: #0f3460;
+    border-bottom-color: #0f3460;
+}
+
+.tab-content {
+    display: none;
+}
+
+.tab-content.active {
+    display: block;
+}
+
+/* Role badge */
+.role-badge {
+    display: inline-block;
+    padding: 3px 10px;
+    border-radius: 12px;
+    font-size: 0.75rem;
+    font-weight: 600;
+    text-transform: uppercase;
+    letter-spacing: 0.3px;
+}
+
+.role-admin {
+    background: #0f3460;
+    color: #fff;
+}
+
+.role-user {
+    background: #e8ebf0;
+    color: #555;
+}
+
+/* Small buttons */
+.btn-small {
+    display: inline-block;
+    padding: 4px 10px;
+    border: none;
+    border-radius: 6px;
+    font-size: 0.78rem;
+    font-weight: 600;
+    cursor: pointer;
+    transition: all 0.15s;
+}
+
+.btn-restore {
+    background: #2ecc71;
+    color: #fff;
+}
+
+.btn-restore:hover {
+    background: #27ae60;
+}
+
+.btn-perm-delete {
+    background: #e74c3c;
+    color: #fff;
+    margin-left: 4px;
+}
+
+.btn-perm-delete:hover {
+    background: #c0392b;
+}
+
+/* Changelog */
+.nowrap {
+    white-space: nowrap;
+}
+
+.text-muted {
+    color: #999;
+    font-size: 0.85rem;
+}