feat(assistant): implement spec 021 chat assistant flow with semantic contracts

frontend/src/lib/components/layout/TopNavbar.svelte

@@ -12,6 +12,8 @@
    * @UX_STATE: SearchFocused -> Search input expands
    * @UX_FEEDBACK: Activity badge shows count of running tasks
    * @UX_RECOVERY: Click outside closes dropdowns
+   * @UX_TEST: SearchFocused -> {focus: search input, expected: focused style class applied}
+   * @UX_TEST: ActivityClick -> {click: activity button, expected: task drawer opens}
    */
 
   import { createEventDispatcher } from "svelte";
@@ -25,6 +27,7 @@
   import { sidebarStore, toggleMobileSidebar } from "$lib/stores/sidebar.js";
   import { t } from "$lib/i18n";
   import { auth } from "$lib/auth/store.js";
+  import { toggleAssistantChat } from "$lib/stores/assistantChat.js";
   import Icon from "$lib/ui/Icon.svelte";
 
   const dispatch = createEventDispatcher();
@@ -64,6 +67,10 @@
     dispatch("activityClick");
   }
 
+  function handleAssistantClick() {
+    toggleAssistantChat();
+  }
+
   function handleSearchFocus() {
     isSearchFocused = true;
   }
@@ -129,6 +136,16 @@
 
   <!-- Nav Actions -->
   <div class="flex items-center gap-3 md:gap-4">
+    <!-- Assistant -->
+    <button
+      class="rounded-lg p-2 text-slate-600 transition-colors hover:bg-slate-100"
+      on:click={handleAssistantClick}
+      aria-label={$t.assistant?.open || "Open assistant"}
+      title={$t.assistant?.title || "AI Assistant"}
+    >
+      <Icon name="activity" size={22} />
+    </button>
+
     <!-- Activity Indicator -->
     <div
       class="relative cursor-pointer rounded-lg p-2 text-slate-600 transition-colors hover:bg-slate-100"