ui: major UX polish and bug fixes for dashboard

- Added global loading spinner and page transitions.
- Improved sidebar with tooltips and persistent collapsed state.
- Fixed chart memory leaks by properly destroying instances on page change.
- Unified WebSocket event handling and status indicators.
- Refined stat cards, tables, and modal interactions.
- Added real backend integration for logout and session management.

static/css/dashboard.css

@@ -403,6 +403,121 @@ body {
     border: 1px solid rgba(255, 255, 255, 0.1);
 }
 
-.ws-dot.connected { background: var(--success); box-shadow: 0 0 8px var(--success); }
-.ws-dot.disconnected { background: var(--danger); }
-.ws-dot.error { background: var(--warning); }
+/* Loading Spinner */
+.spinner-container {
+    display: none;
+    position: fixed;
+    top: 50%;
+    left: 50%;
+    transform: translate(-50%, -50%);
+    z-index: 2000;
+}
+
+.loading .spinner-container {
+    display: block;
+}
+
+.spinner {
+    width: 40px;
+    height: 40px;
+    border: 3px solid rgba(99, 102, 241, 0.1);
+    border-radius: 50%;
+    border-top-color: var(--primary);
+    animation: spin 0.8s linear infinite;
+}
+
+@keyframes spin {
+    to { transform: rotate(360deg); }
+}
+
+/* Page Content Transition */
+.page-content {
+    padding: 2rem;
+    animation: fadeIn 0.3s ease-out;
+}
+
+@keyframes fadeIn {
+    from { opacity: 0; transform: translateY(5px); }
+    to { opacity: 1; transform: translateY(0); }
+}
+
+/* Sidebar Tooltips (for collapsed state) */
+.sidebar.collapsed .menu-item {
+    position: relative;
+}
+
+.sidebar.collapsed .menu-item::after {
+    content: attr(data-tooltip);
+    position: absolute;
+    left: 100%;
+    margin-left: 10px;
+    background: #1e293b;
+    color: white;
+    padding: 4px 8px;
+    border-radius: 4px;
+    font-size: 0.75rem;
+    opacity: 0;
+    pointer-events: none;
+    transition: opacity 0.2s;
+    white-space: nowrap;
+    z-index: 1001;
+}
+
+.sidebar.collapsed .menu-item:hover::after {
+    opacity: 1;
+}
+
+/* Card Header Polish */
+.card-header {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    margin-bottom: 1.25rem;
+}
+
+.card-title {
+    font-size: 1rem;
+    font-weight: 700;
+    color: var(--text-primary);
+    display: flex;
+    align-items: center;
+    gap: 0.5rem;
+}
+
+/* Table Polish */
+.table {
+    width: 100%;
+    border-spacing: 0;
+}
+
+.table th {
+    background: #f8fafc;
+    text-align: left;
+    padding: 0.75rem 1rem;
+    font-size: 0.75rem;
+    font-weight: 600;
+    text-transform: uppercase;
+    letter-spacing: 0.05em;
+    color: #64748b;
+    border-bottom: 1px solid var(--border-color);
+}
+
+.table td {
+    padding: 1rem;
+    border-bottom: 1px solid #f1f5f9;
+}
+
+.table tr:last-child td {
+    border-bottom: none;
+}
+
+/* Stat Card Hover */
+.stat-card {
+    transition: all 0.2s cubic-bezier(0.4, 0, 0.2, 1);
+}
+
+.stat-card:hover {
+    border-color: var(--primary-light);
+    transform: translateY(-2px);
+    box-shadow: var(--shadow-md);
+}