feat: group sessions by project on landing page

- Add loadSessionsAndProjects() to fetch sessions and projects in parallel
- Store projects in window.projectsMap for quick lookup
- Group sessions by projectId, separating assigned and unassigned
- Render collapsible project sections with icon, name, and session count
- Add toggleProjectSection() to collapse/expand sections (▼/▶)
- Display project badges on sessions when assigned to a project
- Unassigned sessions shown in separate section at bottom

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

public/claude-ide/sessions-landing.css

@@ -473,3 +473,138 @@ body.sessions-page {
         max-width: none;
     }
 }
+
+/**
+ * Project Section Styles
+ */
+.project-section {
+    border-bottom: 1px solid #333;
+}
+
+.project-section:last-child {
+    border-bottom: none;
+}
+
+/* Project Section Header */
+.project-section-header {
+    display: flex;
+    align-items: center;
+    gap: 12px;
+    padding: 16px 20px;
+    background: #151515;
+    cursor: pointer;
+    user-select: none;
+    transition: background 0.2s ease;
+}
+
+.project-section-header:hover {
+    background: #1a1a1a;
+}
+
+.project-section-toggle {
+    font-size: 12px;
+    color: #888;
+    transition: transform 0.2s ease;
+    flex-shrink: 0;
+}
+
+.project-section-icon {
+    font-size: 20px;
+    flex-shrink: 0;
+}
+
+.project-section-name {
+    font-size: 15px;
+    font-weight: 600;
+    color: #e0e0e0;
+    flex: 1;
+}
+
+.project-section-count {
+    font-size: 13px;
+    color: #888;
+    font-weight: 500;
+    flex-shrink: 0;
+}
+
+/* Project Section Sessions Container */
+.project-section-sessions {
+    display: block;
+    transition: all 0.3s ease;
+}
+
+.project-section.collapsed .project-section-sessions {
+    display: none;
+}
+
+/* Session Rows within Project Sections */
+.session-row {
+    border-bottom: 1px solid #252525;
+    transition: background 0.2s ease;
+    cursor: pointer;
+}
+
+.session-row:last-child {
+    border-bottom: none;
+}
+
+.session-row:hover {
+    background: #252525;
+}
+
+/* Session Project Badge */
+.session-project-badge {
+    display: inline-block;
+    margin-top: 4px;
+    padding: 3px 8px;
+    background: rgba(74, 158, 255, 0.15);
+    color: #4a9eff;
+    border-radius: 4px;
+    font-size: 11px;
+    font-weight: 600;
+    text-transform: uppercase;
+    letter-spacing: 0.5px;
+}
+
+/* Unassigned Section Special Styling */
+.unassigned-section .project-section-header {
+    background: #1a1a1a;
+    border-top: 1px solid #333;
+}
+
+.unassigned-section .project-section-header:hover {
+    background: #222;
+}
+
+/* Collapsed state */
+.project-section.collapsed .project-section-toggle {
+    transform: rotate(-90deg);
+}
+
+/* Responsive Design for Project Sections */
+@media (max-width: 768px) {
+    .project-section-header {
+        padding: 14px 16px;
+    }
+
+    .project-section-toggle {
+        font-size: 11px;
+    }
+
+    .project-section-icon {
+        font-size: 18px;
+    }
+
+    .project-section-name {
+        font-size: 14px;
+    }
+
+    .project-section-count {
+        font-size: 12px;
+    }
+
+    .session-project-badge {
+        font-size: 10px;
+        padding: 2px 6px;
+    }
+}