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/package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "rfc-app-frontend",
-  "version": "0.1.0",
+  "version": "0.2.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "rfc-app-frontend",
-      "version": "0.1.0",
+      "version": "0.2.1",
       "dependencies": {
         "@codemirror/commands": "^6.10.3",
         "@codemirror/lang-markdown": "^6.5.0",