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/components/AppHeader.vue

@@ -45,7 +45,8 @@ router.afterEach(() => {
       <!-- Center: primary navigation (desktop) -->
       <div class="nav-center">
         <div class="nav-pill-bar">
-          <router-link to="/knowledge" class="nav-link" :class="{ 'router-link-active': isKnowledgeActive }">Knowledge</router-link>
+          <router-link to="/dashboard" class="nav-link">Dashboard</router-link>
+          <router-link to="/knowledge" class="nav-link" :class="{ 'router-link-active': isKnowledgeActive }">Browse</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="/rules" class="nav-link">Rulebooks</router-link>
@@ -90,7 +91,8 @@ router.afterEach(() => {
 
     <!-- Mobile dropdown -->
     <div v-if="mobileMenuOpen" class="mobile-menu">
-      <router-link to="/knowledge" class="nav-link" :class="{ 'router-link-active': isKnowledgeActive }">Knowledge</router-link>
+      <router-link to="/dashboard" class="nav-link">Dashboard</router-link>
+      <router-link to="/knowledge" class="nav-link" :class="{ 'router-link-active': isKnowledgeActive }">Browse</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="/rules" class="nav-link">Rulebooks</router-link>