Release 0.2.1: PRView Rules-of-Hooks fix (React #310 blank page)

PRView.jsx declared its threadsByKind useMemo after the early-return
guard for the loading state. First mount: pr is null, early return
fires, 14 hooks called. Second render: pr populated, execution
reaches the useMemo, 15 hooks — React #310, page goes blank.

Move the useMemo above the early returns and null-safe the
pr?.threads access so the hook count stays stable across renders.

Pre-existing bug in the post-Contribute-rewrite PR view; surfaced
in production by 0.2.0's graduation merge race fix enabling the
workflow that opens this code path. Patch per §20.2 — no operator
action required beyond rebuilding the frontend and restarting.

frontend/src/components/PRView.jsx

@@ -143,6 +143,19 @@ export default function PRView({ viewer }) {
     }
   }, [slug, prNumber, reviewText, reviewDraft, refresh])
 
+  // §10.6: split messages by thread kind for the visual distinction
+  // §10.4 requires. Within each kind, sort by id for chronological
+  // order. Declared above the early returns below so the hook count
+  // stays stable between the loading render (pr == null) and the
+  // loaded render (Rules of Hooks — React #310 otherwise).
+  const threadsByKind = useMemo(() => {
+    const out = { chat: [], review: [], flag: [] }
+    for (const t of pr?.threads || []) {
+      out[t.thread_kind]?.push(t)
+    }
+    return out
+  }, [pr?.threads])
+
   // ── render ─────────────────────────────────────────────────────
   if (error) return <article className="entry-view"><p>Error: {error}</p></article>
   if (!pr) return <article className="entry-view">Loading PR…</article>
@@ -154,17 +167,6 @@ export default function PRView({ viewer }) {
   const supersededBy = pr.superseded_by_pr_number
   const supersedes = pr.supersedes_pr_number
 
-  // §10.6: split messages by thread kind for the visual distinction
-  // §10.4 requires. Within each kind, sort by id for chronological
-  // order.
-  const threadsByKind = useMemo(() => {
-    const out = { chat: [], review: [], flag: [] }
-    for (const t of pr.threads || []) {
-      out[t.thread_kind]?.push(t)
-    }
-    return out
-  }, [pr.threads])
-
   const seenSha = pr.seen?.last_seen_commit_sha
   const seenMsgId = pr.seen?.last_seen_message_id || 0