Home: sign the brief save on Replace too (Codex polish)

Codex spotted the one remaining unsigned BRIEF_VIEW_KEY write — in replaceArticle.
Not a safety issue (instant-paint requires cached.sig, so an unsigned entry just
won't instant-paint), but it meant the next load after a Replace fell back to
"Gathering…" until a fresh /api/brief re-saved the signed version. Now every
brief save includes sig: briefSig() (computed after dismissed updates), so an
edited brief still instant-paints. All three save paths verified signed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/+page.svelte

@@ -424,7 +424,9 @@
       if (i >= 0) {
         brief.items[i] = repl;
         brief = { ...brief, items: [...brief.items] };
-        P.saveJSON(BRIEF_VIEW_KEY, { generated_at: brief.generated_at, items: brief.items });
+        // Sign the save (dismissed just changed) so the next load can still
+        // instant-paint this edited brief instead of falling back to "Gathering…".
+        P.saveJSON(BRIEF_VIEW_KEY, { generated_at: brief.generated_at, items: brief.items, sig: briefSig() });
       }
     } else {
       const i = feed.findIndex((a) => a.id === article.id);