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/todo.md

@@ -106,7 +106,7 @@ const _kpiBox = html`<div class="kpi-infobox">
     </div>
   </div>
   <div class="kpi-row">
-    <span class="kpi-row-label">improvements (open)</span>
+    <span class="kpi-row-label">suggestions (open)</span>
     <div class="kpi-row-right">
       <div class="kpi-row-value" style="color:${improvements.length > 0 ? '#6366f1' : 'inherit'}">${improvements.length}</div>
     </div>
@@ -209,14 +209,14 @@ if (thirdParty.length === 0) {
 
 ---
 
-## Improvements
+## Suggestions
 
 Dashboard suggestions submitted via Shift+click. Review and action on the
 [UI Feedback](/ui-feedback) page; open items shown here for visibility.
 
 ```js
 if (improvements.length === 0) {
-  display(html`<p class="dim">No open improvement suggestions. Shift+click any widget to submit one.</p>`);
+  display(html`<p class="dim">No open suggestions. Shift+click any widget to submit one.</p>`);
 } else {
   display(html`<div class="task-list">${improvements.map(t => html`
     <div class="task-item impr-item">