Task 13: Build CareerActivity tile

Created CareerActivityTile component with full timeline merged from multiple data sources:
- Builds 10 activity entries matching the concept HTML spec exactly
- Color-coded dots by type: role (teal), project (amber), cert (green), edu (purple)
- Two-column responsive grid (1 column below 900px)
- Entry types: 4 roles, 2 projects, 3 certifications, 1 education
- Data sources: consultations, investigations, documents
- Sorted newest-first with stable ordering for same-year entries
- Added .activity-grid responsive CSS class to index.css
- Wired into DashboardLayout below LastConsultationTile

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

src/index.css

@@ -276,3 +276,16 @@ html {
     grid-template-columns: 1fr;
   }
 }
+
+/* Activity grid responsive */
+.activity-grid {
+  display: grid;
+  grid-template-columns: 1fr 1fr;
+  gap: 10px;
+}
+
+@media (max-width: 900px) {
+  .activity-grid {
+    grid-template-columns: 1fr;
+  }
+}