feat: add /news archive view with feed filter and pagination

- NewsView.vue: 90-day article archive, feed filter dropdown,
  load-more pagination, topic pills, thumbs up/down reactions
- Router: add /news route
- AppHeader: add News nav link (desktop + mobile)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/AppHeader.vue

@@ -79,6 +79,7 @@ router.afterEach(() => {
         <router-link to="/graph" class="nav-link">Graph</router-link>
         <router-link to="/calendar" class="nav-link">Calendar</router-link>
         <router-link to="/briefing" class="nav-link">Briefing</router-link>
+        <router-link to="/news" class="nav-link">News</router-link>
         <router-link to="/shared" class="nav-link">Shared</router-link>
       </div>
 
@@ -129,6 +130,7 @@ router.afterEach(() => {
       <router-link to="/graph" class="nav-link">Graph</router-link>
       <router-link to="/calendar" class="nav-link">Calendar</router-link>
       <router-link to="/briefing" class="nav-link">Briefing</router-link>
+      <router-link to="/news" class="nav-link">News</router-link>
       <router-link to="/shared" class="nav-link">Shared</router-link>
       <div class="mobile-divider"></div>
       <router-link to="/settings" class="nav-link">Settings</router-link>