Phase 4: Rich card features — labels, checklists, comments, attachments, categories

Add full CRUD + UI for all card sub-resources (tags/labels, assignees, checklists
with items, comments with markdown, file attachments). Restructure CardModal into
a two-column layout with lazy-loaded data. Upgrade card thumbnails with named label
pills, urgency-colored due date badges, checklist progress indicators, and SVG icons.
Add board categories with filtering on the listing page. Include markdown rendering
with DOMPurify sanitization and .prose CSS styles.

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

package.json

@@ -22,8 +22,10 @@
 		"@prisma/client": "^6.6.0",
 		"@sveltejs/adapter-node": "^5.2.0",
 		"@sveltejs/kit": "^2.50.2",
+		"@types/dompurify": "^3.0.5",
 		"bcrypt": "^6.0.0",
 		"date-fns": "^4.1.0",
+		"dompurify": "^3.3.1",
 		"express": "^5.1.0",
 		"marked": "^15.0.0",
 		"socket.io": "^4.8.0",