fix(#290 review): drop stale cached children on boot + gate size warn (F1/F2/F3)

F1 (MEDIUM regression): a collapsed-but-cached branch showed STALE children on
re-expand after reload (the cache keeps children of any ever-expanded branch;
refreshOpenBranches only refreshes OPEN branches, but the fetch guard skips a branch
that has cached children). New pruneCollapsedChildren(tree, openIds) resets children
to [] (keeps hasChildren) for every node NOT in the persisted open-set, recursing
into open nodes — a once-per-mount boot effect. A pruned collapsed branch is then the
'unloaded' shape handleToggle re-fetches, so its first expand reconciles fresh (as
pre-cache). Open branches keep their children (refreshOpenBranches handles them, no
double fetch). Test: a collapsed cached branch with a stale child fetches fresh on
first expand after boot.
F2: gate the >4MB size-guard console.warn behind the writeFailureWarned once-flag
(like the quota branch) so editing a huge tree no longer re-warns every ~500ms; test
that an oversized tree is not persisted + warns exactly once.
F3: narrow the use-auth privacy comment (only tree caches are swept; other
localStorage entries remain).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/auth/hooks/use-auth.ts

@@ -123,8 +123,10 @@ export default function useAuth() {
 
   const handleLogout = async () => {
     setCurrentUser(RESET);
-    // Purge the persisted sidebar tree caches (they contain page titles) so
-    // nothing readable is left in localStorage on a shared machine.
+    // Purge the persisted sidebar tree caches (they contain page titles) so the
+    // cached page titles aren't left readable in localStorage on a shared
+    // machine. (Only the tree caches are swept; other localStorage entries
+    // remain.)
     clearPersistedTreeCaches();
     await logout();
     window.location.replace(`${APP_ROUTE.AUTH.LOGIN}?logout=1`);