fix: briefing discuss button — full article content + scroll + optimistic UI

Three bugs in discussArticle():
- Scroll selector was '.briefing-chat' (doesn't exist) → '.briefing-center';
  the panel never scrolled into view so the response was invisible until refresh
- Only 300-char snippet was sent to the LLM; now passes the full stored
  content (up to 2000 chars) from the backend
- User message wasn't shown until streaming ended; now added optimistically
  to messages[] immediately on click so it appears straight away

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

frontend/src/types/news.ts

@@ -3,6 +3,7 @@ export interface NewsItem {
   title: string
   url: string
   snippet: string
+  content: string
   published_at: string | null
   topics: string[]
   source: string

frontend/src/views/BriefingView.vue

@@ -176,12 +176,25 @@ async function send(overrideText?: string) {
 }
 
 async function discussArticle(item: NewsItem) {
-  const snippet = item.snippet ? `\n\n"${item.snippet.slice(0, 400)}${item.snippet.length > 400 ? '…' : ''}"` : ''
-  const text = `Summarize the following article excerpt for me in a few sentences and share your thoughts. Do not use any research or search tools — just respond conversationally based on what I've shared below.\n\n**${item.title}**${snippet}\n\nSource: ${item.source}`
-  // Scroll the chat panel into view on narrow layouts, then send
+  const body = (item.content || item.snippet || '').trim()
+  const bodyBlock = body ? `\n\n---\n${body}\n---` : ''
+  const text = `Please summarize and share your thoughts on this article. Do not use any research or search tools — respond conversationally based only on the content below.\n\n**${item.title}**\nSource: ${item.source}${bodyBlock}`
+
+  // Add the user message optimistically so it appears immediately
+  const optimisticMsg: BriefingMessage = {
+    id: Date.now(),
+    role: 'user',
+    content: text,
+    created_at: new Date().toISOString(),
+  }
+  messages.value = [...messages.value, optimisticMsg]
+
+  // Scroll the chat panel into view
   await nextTick(() => {
-    document.querySelector('.briefing-chat')?.scrollIntoView({ behavior: 'smooth', block: 'nearest' })
+    document.querySelector('.briefing-center')?.scrollIntoView({ behavior: 'smooth', block: 'nearest' })
+    scrollToBottom()
   })
+
   await send(text)
 }