feat: compact dashboard + login system for admin pages

Dashboard:
- Much smaller cards (col-xl-2), compact badges
- Short labels (Buero, HO, DND, OoO, etc.)
- Optimized for small screens

Login:
- Password-protected admin pages (all except dashboard)
- Dashboard remains public (read-only status view)
- Login page with redirect back to requested page
- Session cookie (24h, regenerated on restart)
- Password set via Settings page (SHA256 hashed)
- Sidebar shows only Dashboard when not logged in
- Login/Logout link in sidebar footer

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/style.css

@@ -119,6 +119,45 @@ body {
     vertical-align: middle;
 }
 
+/* Compact dashboard */
+.seg-compact {
+    padding: 4px 8px;
+    border-radius: 4px;
+    margin-bottom: 3px;
+    font-size: 0.8rem;
+}
+
+.seg-compact .seg-name {
+    display: block;
+    font-weight: 500;
+    white-space: nowrap;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    font-size: 0.78rem;
+}
+
+.seg-compact .seg-badges {
+    display: flex;
+    gap: 3px;
+    flex-wrap: wrap;
+    margin-top: 2px;
+}
+
+.seg-compact.seg-empty {
+    background: #f8f8f8;
+    border-left: 3px solid #ddd;
+}
+
+.badge-sm {
+    font-size: 0.65rem;
+    padding: 1px 5px;
+    border-radius: 3px;
+    font-weight: 500;
+    color: #fff;
+    display: inline-block;
+    line-height: 1.4;
+}
+
 @media (max-width: 768px) {
     .sidebar {
         width: 100%;