Make dismissed reactive ($state) to clear the Svelte build warning

dismissed.size is read in the template (the History 'Clear' control), so the Set
must be $state for Svelte 5 to track .add()/reassignment. Build is warning-clean
again. Frontend only — rebuild + refresh.

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

frontend/src/routes/+page.svelte

@@ -26,8 +26,8 @@
   const BRIEF_VIEW_KEY = 'goodnews:brief_view'; // {date, items} — the reader's curated brief
   const HISTORY_CAP = 200;
 
-  let seenIds = new Set();
-  let dismissed = new Set();
+  let seenIds = new Set(); // handler-only, not rendered — no reactivity needed
+  let dismissed = $state(new Set()); // read in the template (Clear button) — must be reactive
   let history = $state([]);
 
   function persistSession() {