refactor(frontend): declare props read-only, and drop the deprecated antd prop (#100)

Seventeen components declared props the compiler was free to assume were mutable, and one antd prop had gone stale. Both mechanical, neither with any behaviour attached.

React never writes to props, and `Readonly<>` says so to the compiler rather than only to the reader. This finishes a pattern the codebase had already chosen rather than introducing one: AccountDetails and EmailTemplateEditor were already written as `type Props = Readonly<{…}>`, so the thirteen named prop interfaces are converted to that same shape and the four context providers, which annotate `{ children }` inline, get `Readonly<{ children: React.ReactNode }>`.

Cart.tsx was the last place passing `destroyOnClose`, deprecated in antd 5.20. Twelve other call sites across the admin screens, the filter drawer and four customer modals already use `destroyOnHidden`, so this one was simply stale. Deprecated props keep working until they do not, and the failure then arrives as an antd upgrade breaking something unrelated to the change being made.

Counted rather than assumed, which the issue specifically asks for, because a `Readonly<>` in the wrong position type-checks and fixes nothing: lint goes from 31 warnings to 13, a drop of exactly eighteen, and both rules disappear from the breakdown entirely rather than merely thinning out.

What that leaves is the point of doing it. The remaining thirteen are eleven `set-state-in-effect` and two `no-alphabetical-sort` — so the frontend's warnings are now only the ones that need a decision, which is what makes #99 tractable. It had grown from the eight in that issue's title to eleven, two of them added by #97's clock tick and lapsed-cart refetch.

No behaviour change intended, so the bar was the end-to-end suite. Full run: 121 passed, 8 failed; all eight pass in a 45/45 serial re-run, which is the shared-database and event-loop flakiness this suite has had throughout.

Closes #100

frontend/src/customer/AuthForm.tsx

@@ -23,7 +23,7 @@ export type AuthMode = 'register' | 'login';
 export const MARKETING_CONSENT_TEXT =
   'I want to receive occasional emails about new one-of-a-kind items from Redefined Designs. I can unsubscribe at any time.';
 
-interface Props {
+type Props = Readonly<{
   mode: AuthMode;
   onModeChange: (mode: AuthMode) => void;
   onForgotPassword: () => void;
@@ -31,7 +31,7 @@ interface Props {
   // route closes back to the page behind it, while the cart and favorite
   // prompts resume the action the customer was interrupted doing.
   onSuccess: () => void;
-}
+}>;
 
 // The one implementation of signing in and registering. It was previously
 // written twice — once as the /login and /register pages, once inside the