fix(dashboard): domain field name in TD payload; rename Improvements → Suggestions

- improvement-modal.js: API expects `domain` not `domain_slug` (422 fix)
- todo.md: section heading and KPI label renamed to "Suggestions"

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

dashboard/src/components/improvement-modal.js

@@ -337,7 +337,7 @@ export function initImprovementModal({ apiBase = "http://127.0.0.1:8000", domain
 
       const location = `${pageName} › ${widgetName}`;
       const payload = {
-        domain_slug: domain,
+        domain: domain,
         title: `UI: ${widgetName}`,
         description: suggestion,
         debt_type: "dashboard-improvement",