chore(ui): bump the main.js cache-bust stamp to v6

web/static/*.js is gitignored and built at image-build time, so the
hardcoded ?v= on the module script tag is the ONLY mechanism that
evicts a stale bundle from browsers. Shipping the series twin deletion
without a bump would let cached main.js (with the deleted renderer)
keep running on prod, making the paginator fix appear not to work.
Found while shipping; easy to miss because the bundle never appears in
git status.

templates/header.templ

@@ -241,7 +241,7 @@ templ Header(user User, currentPath string) {
 		</header>
 		<!-- module bundle; ?v=<commit-ish stamp> busts stale intermediate
 		     builds across app restarts (browser caches are keyed by URL) -->
-		<script type="module" src="/static/main.js?v=5"></script>
+		<script type="module" src="/static/main.js?v=6"></script>
 	</div>
 }