feat(dashboard): right-click improvement modal + UI Feedback page

- improvement-modal.js: global contextmenu handler that opens a modal
  showing page/widget context; submits suggestions as TD items with
  debt_type="dashboard-improvement" to POST /technical-debt/
- _footer.md: shared Observable footer that auto-initialises the modal
  on every dashboard page
- ui-feedback.md: review/approval page — lists open suggestions with
  resolve / won't-fix / in-progress action buttons; archived items shown
  below; live-polled
- observablehq.config.js: "UI Feedback" added under Workstreams group

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

dashboard/observablehq.config.js

@@ -19,6 +19,7 @@ export default {
         { name: "Tasks",         path: "/tasks" },
         { name: "Interventions", path: "/interventions" },
         { name: "Debt",          path: "/techdept" },
+        { name: "UI Feedback",   path: "/ui-feedback" },
         { name: "Extends",       path: "/extensions" },
         { name: "Dependencies",  path: "/dependencies" },
       ],