fix(sw.js): bump CACHE version to force clients off the poisoned cache

The service worker caches app.js/api.js cache-first under a static
key. Editing those files alone does not force a refetch -- only
changing the service-worker script itself does. That let browsers
that had visited before the two syntax-error commits keep serving
old-but-working cached JS indefinitely, while new visitors got the
broken files straight from the network.

Bumping bharatgraph-v5 -> v6 invalidates the old cache on next visit.
Still requires one bump per deploy going forward; a content-hashed
cache name would remove that manual step as a follow-up.

frontend/sw.js

@@ -3,7 +3,7 @@
 // NEW-A5 FIX: host === "hf.space" never matches -- HF uses *.hf.space subdomains
 // SW-01 FIX: use relative paths so caching works on GitHub Pages
 
-const CACHE = "bharatgraph-v5";
+const CACHE = "bharatgraph-v6";
 const STATIC = [
   "./",
   "./index.html",