Fix pack store: don't cache empty fallback results

Two bugs caused the empty packs page:

1. When the API failed, catchError returned [] and writeCache() stored
   it in sessionStorage. On next visit, readCache() returned [] (truthy),
   so the store never re-fetched from the API — stuck on empty forever.

2. Fix: only cache successful non-empty API responses. Treat empty
   cached arrays as cache misses so the store always re-fetches.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/Web/StellaOps.Web/src/app/features/policy-studio/services/policy-pack.store.ts

@@ -74,7 +74,9 @@ export class PolicyPackStore {
       .subscribe((packs) => {
         this.usingFallback = fetchedFromFallback;
         this.packs$.next(packs);
-        this.writeCache(packs);
+        if (!fetchedFromFallback && packs.length > 0) {
+          this.writeCache(packs);
+        }
       });
   }
 
@@ -95,7 +97,8 @@ export class PolicyPackStore {
     try {
       const raw = sessionStorage.getItem(this.cacheKey);
       if (!raw) return null;
-      return JSON.parse(raw) as PolicyPackSummary[];
+      const parsed = JSON.parse(raw) as PolicyPackSummary[];
+      return Array.isArray(parsed) && parsed.length > 0 ? parsed : null;
     } catch {
       return null;
     }