feat(illness): episode detail, retrospective surface, edit + delete guard

Add an episode-detail page at /illness/[id] mirroring the lab-biomarker detail:
it renders the per-episode correlation card (recovery gap, red flags, pre-onset
and nadir findings, or 'still learning' when the signal is thin) and surfaces
the stored note. List rows link into it; the cross-episode retrospective summary
sits below the history. Fold the row's inline buttons into a shared overflow
menu and gate delete behind a confirmation dialog. Let the new-episode sheet
double as an edit form, add the FLARE course and a parent-condition picker, and
let the day-log sheet backdate within the episode window. Swap the loading and
empty states onto the shared primitives.

src/app/illness/[id]/page.tsx

@@ -0,0 +1,56 @@
+"use client";
+
+/**
+ * v1.18.1 — per-episode detail page (`/illness/[id]`), mirroring
+ * `/labs/[biomarkerId]`. Born-gated on the resolved `modules.illness` flag;
+ * an unauthenticated visitor is bounced to login, an account without the
+ * module opted in is bounced home (every `/api/illness/*` route also enforces
+ * the gate server-side, so this is a UX redirect, not the security boundary).
+ */
+import { use, useEffect } from "react";
+import { useRouter } from "next/navigation";
+import { Loader2 } from "lucide-react";
+
+import { IllnessEpisodeDetail } from "@/components/illness/illness-episode-detail";
+import { BackLink } from "@/components/ui/back-link";
+import { useAuth } from "@/hooks/use-auth";
+import { useMounted } from "@/hooks/use-mounted";
+import { useTranslations } from "@/lib/i18n/context";
+
+export default function IllnessEpisodePage({
+  params,
+}: {
+  params: Promise<{ id: string }>;
+}) {
+  const { id } = use(params);
+  const { user, isAuthenticated, isLoading } = useAuth();
+  const mounted = useMounted();
+  const router = useRouter();
+  const { t } = useTranslations();
+
+  const enabled = user?.modules?.illness === true;
+
+  useEffect(() => {
+    if (isLoading) return;
+    if (!isAuthenticated) {
+      router.push("/auth/login");
+    } else if (!enabled) {
+      router.push("/");
+    }
+  }, [isLoading, isAuthenticated, enabled, router]);
+
+  if (!mounted || isLoading || !isAuthenticated || !enabled) {
+    return (
+      <div className="flex h-64 items-center justify-center">
+        <Loader2 className="text-primary h-8 w-8 animate-spin motion-reduce:animate-none" />
+      </div>
+    );
+  }
+
+  return (
+    <div className="space-y-6">
+      <BackLink href="/illness" label={t("illness.backToList")} />
+      <IllnessEpisodeDetail episodeId={id} />
+    </div>
+  );
+}