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/static/panel.css

@@ -245,6 +245,11 @@ form.inline { display: inline; margin: 0; }
 main.page-login, main.page-setup { max-width: 24rem; }
 .card + .card { margin-top: 1.2rem; }
 .flash { background: var(--flash-bg); border: 1px solid var(--flash-border); color: var(--flash-fg); padding: 0.7rem 1rem; border-radius: 6px; margin-bottom: 1.2rem; }
+/* RateLimitErr renders as .flash.error: a validation failure, not a success
+   notice. Without this rule it inherited the success surface (green) and only
+   .error's text colour, so a rejected rate limit read as good news in red
+   text. Same box, danger palette. */
+.flash.error { background: var(--danger-bg); border-color: var(--danger-border); color: var(--danger-fg); }
 table { width: 100%; border-collapse: collapse; }
 /* A table column is at least as wide as the longest unbreakable run inside it,
    and the panel's tables are full of runs with nothing to break on: email