feat(design-system): add design review PR template with screenshot requirement

.github/pull_request_template.md

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+## Summary
+
+<!-- Brief description of the change -->
+
+## Screenshots
+
+<!-- REQUIRED for any UI change. Before/after screenshots. -->
+
+| Before              | After               |
+| ------------------- | ------------------- |
+| <!-- screenshot --> | <!-- screenshot --> |
+
+## Checklist
+
+- [ ] Design tokens used (no hardcoded hex)
+- [ ] Focus-visible works (keyboard nav)
+- [ ] ARIA labels on icon-only buttons
+- [ ] Empty state handled for list views
+- [ ] Confirmation dialog on destructive actions
+- [ ] Loading state on async operations
+- [ ] Dark/light theme tested