docs(web-ui): deny dialog uses themedPrompt textarea, not window.prompt

The deny-reason dialog moved from window.prompt() to a themed resizable
textarea (Enter submits, Shift+Enter newline). Update the dashboard route
reference that still described the old single-line window.prompt() flow.

Closes #1849.

docs/web-ui/dashboard.md

@@ -979,7 +979,10 @@ that's a browser-level decision, not ours.
   approval with an optional operator-supplied reason. The reason
   travels to the manager as `HelperEvent::ApprovalResolved.note`
   and also rides on the dashboard's `ApprovalResolved` event.
-  Dashboard prompts via `window.prompt()` on click.
+  Dashboard prompts via the themed `themedPrompt()` dialog on
+  click — a resizable `<textarea>` where Enter submits and
+  Shift+Enter inserts a newline (so multi-sentence rejection
+  notes are possible).
 - `POST /{rebuild,kill,restart,start,destroy}/{name}` — lifecycle.
   `destroy` accepts `purge=on` to also wipe state dirs.
 - `POST /purge-tombstone/{name}` — wipe a tombstone's state dirs.