fix: quality audit — memory leaks, dead prose classes, stale state scope

- safe-markdown: add `cancelled` flag to async useEffect; no more
  setState after unmount when component is destroyed mid-import
- rich-editor: same cleanup for preview tab effect; also remove dead
  `prose prose-sm max-w-none` classes (Tailwind Typography not installed)
  and replace with explicit arbitrary-selector markdown styling that
  actually renders using existing design tokens
- profile-certificate-slot: drop dead `ORDER_LABEL`, `order` variable,
  and `physicalOrder` from `MineCertificateResponse`; physical orders
  are disabled — these were leftover references causing lint noise
- learning-panel: change `issuingCert: boolean` → `issuingCertFor:
  number | null` so only the targeted user's button shows a spinner;
  a boolean shared across all users caused the wrong row to animate
  when the admin switched selected users mid-request

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/profile-certificate-slot.tsx

@@ -5,7 +5,7 @@ import { useRouter } from "next/navigation";
 import { useQuery, useQueryClient } from "@tanstack/react-query";
 import { Award, Clock, Copy, ExternalLink, Loader2, Lock } from "lucide-react";
 import { http, toUserError } from "@/lib/api";
-import type { Certificate, CertificatePhysicalOrder } from "@/lib/types";
+import type { Certificate } from "@/lib/types";
 import { Button } from "@/components/ui/button";
 import { toast } from "@/components/providers";
 import { cn } from "@/lib/utils";
@@ -13,14 +13,6 @@ import { cn } from "@/lib/utils";
 type MineCertificateResponse = {
   certificate: Certificate | null;
   eligible: boolean;
-  physicalOrder: CertificatePhysicalOrder | null;
-};
-
-const ORDER_LABEL: Record<string, string> = {
-  requested: "درخواست فیزیکی ثبت شد",
-  paid: "پرداخت تأیید شد",
-  shipped: "ارسال شد",
-  cancelled: "درخواست فیزیکی لغو شده",
 };
 
 export function ProfileCertificateSlot() {
@@ -36,7 +28,6 @@ export function ProfileCertificateSlot() {
 
   const cert = data?.certificate ?? null;
   const eligible = !!data?.eligible;
-  const order = data?.physicalOrder;
 
   // Auto-issue once when the student becomes eligible (same path as /cap).
   useEffect(() => {