fix(panel): GUI defects from the 1.2.x layout pass (P3)

- .flash.error now renders on the danger surface instead of the success
  one; RateLimitErr previously showed as green with red text.
- User delete goes through a confirmation page (GET/POST
  /users/{uid}/delete), matching the domain-delete pattern, instead of a
  plain submit button next to Save with no confirmation.
- Extracted the repeated DNS Host/Type/Value markup on a domain's page and
  the duplicated Settings credentials form into shared partials. No
  behaviour change.

docs/plans/code-review.md P3 checked off; CHANGELOG updated.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/web/view/templates_test.go

@@ -282,6 +282,7 @@ func TestSettingsPageDocumentsRateLimits(t *testing.T) {
 func TestDrillDownPagesPlaceBackLinkAboveContent(t *testing.T) {
 	drillDown := map[string]bool{
 		"user_form.html":     true,
+		"user_delete.html":   true,
 		"domain_detail.html": true,
 		"domain_delete.html": true,
 		"delivery.html":      true,