feat(dashboard): DashboardView landing + route + nav

Task 3 of #583. New DashboardView at /dashboard composes the approved layout:
done-recently strip, Active-now project panels (project -> active milestones ->
open tasks, in-progress flagged, + no-milestone group), and a rail with
upcoming events / week stats / quick-create (Task/Note/Process). '/' now
redirects to /dashboard; AppHeader gains a Dashboard link and relabels
Knowledge -> Browse (route unchanged). Empty + loading states included.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/router/index.ts

@@ -5,10 +5,15 @@ const router = createRouter({
   history: createWebHistory(),
   routes: [
     {
-      // Knowledge is the landing page in the MCP-first architecture
-      // (chat / journal / workspace surfaces have been removed).
+      // The dashboard ("what to work on") is the landing page; Knowledge
+      // remains as the exhaustive "Browse" surface.
       path: "/",
-      redirect: "/knowledge",
+      redirect: "/dashboard",
+    },
+    {
+      path: "/dashboard",
+      name: "dashboard",
+      component: () => import("@/views/DashboardView.vue"),
     },
     {
       path: "/knowledge",