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>

src/routes/api/boards/+server.ts

@@ -50,7 +50,7 @@ export const POST: RequestHandler = async ({ request, locals }) => {
 		return json({ error: result.error.issues[0].message }, { status: 400 });
 	}
 
-	const { title, description, visibility } = result.data;
+	const { title, description, visibility, categoryId } = result.data;
 
 	const board = await withTenant(tenant.id, async (tx) => {
 		return tx.board.create({
@@ -59,6 +59,7 @@ export const POST: RequestHandler = async ({ request, locals }) => {
 				title,
 				description,
 				visibility,
+				...(categoryId ? { categoryId } : {}),
 				members: {
 					create: {
 						userId: locals.user!.id,