feat: UI Problems list filters, detail card and event timeline (d2-1)

- Filters status/severity/hostname; /problems/:id with Ack/Resolve and timeline

- Dashboard drill-down to open problems

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/views/DashboardView.vue

@@ -12,7 +12,7 @@
       <div class="dash-card">
         <div class="dash-value">{{ data.problems_open }}</div>
         <div class="dash-label">Открытых проблем</div>
-        <RouterLink to="/problems">Проблемы →</RouterLink>
+        <RouterLink :to="{ path: '/problems', query: { status: 'open' } }">Проблемы →</RouterLink>
       </div>
       <div class="dash-card">
         <div class="dash-value">{{ data.hosts_total }}</div>