feat(journal): add JournalView UI with /journal route + nav link

Restores a working browser surface for the journal feature, which was
left UI-less when Stage F of the original plan was deferred. JournalView
is a fresh write (not a rename of BriefingView) — drops the briefing-
specific weather panel, news cards, RSS reactions, article-discuss
button, and setup wizard, since none of those have journal-backend
equivalents.

What it does:
- Fetches /api/journal/today on mount; shows today's daily-prep card
  (rendered from msg_metadata.kind === 'daily_prep' sections)
- Day picker via /api/journal/days lets you switch to past days
- Refresh-prep button hits /api/journal/trigger-prep
- Center is the existing ChatPanel pinned to today's journal conversation
  (so the chat input + SSE + tool-call cards inherit unchanged)
- Right sidebar keeps the upcoming-events list (uses the generic
  /api/events endpoint, not a briefing-specific one)

Also:
- Replace the dead Briefing API client functions with Journal equivalents
  (getJournalConfig, saveJournalConfig, getJournalToday, getJournalDay,
  getJournalDays, triggerJournalPrep, list/update/deleteJournalMoment).
- Remove NewsView.vue — it was orphaned (no route, no nav) and depended
  on the deleted /api/briefing/* endpoints. If a standalone news surface
  is wanted later it'll need to be rebuilt against new endpoints.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/components/AppHeader.vue

@@ -76,6 +76,7 @@ router.afterEach(() => {
         <div class="nav-pill-bar">
           <router-link to="/" class="nav-link" :class="{ 'router-link-active': isKnowledgeActive }">Knowledge</router-link>
           <router-link to="/chat" :class="['nav-link', { 'router-link-active': isChatActive }]">Chat</router-link>
+          <router-link to="/journal" class="nav-link">Journal</router-link>
           <router-link to="/calendar" class="nav-link">Calendar</router-link>
           <router-link to="/projects" class="nav-link">Projects</router-link>
         </div>
@@ -123,6 +124,7 @@ router.afterEach(() => {
     <div v-if="mobileMenuOpen" class="mobile-menu">
       <router-link to="/" class="nav-link" :class="{ 'router-link-active': isKnowledgeActive }">Knowledge</router-link>
       <router-link to="/chat" :class="['nav-link', { 'router-link-active': isChatActive }]">Chat</router-link>
+      <router-link to="/journal" class="nav-link">Journal</router-link>
       <router-link to="/calendar" class="nav-link">Calendar</router-link>
       <router-link to="/projects" class="nav-link">Projects</router-link>
       <router-link to="/shared" class="nav-link">Shared</router-link>