feat(assistant): add multi-dialog UX, task-aware llm settings, and i18n cleanup

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

@@ -29,6 +29,7 @@
   import { auth } from "$lib/auth/store.js";
   import { toggleAssistantChat } from "$lib/stores/assistantChat.js";
   import Icon from "$lib/ui/Icon.svelte";
+  import LanguageSwitcher from "$lib/ui/LanguageSwitcher.svelte";
 
   const dispatch = createEventDispatcher();
 
@@ -105,7 +106,7 @@
     <button
       class="rounded-lg p-2 text-slate-600 transition-colors hover:bg-slate-100 md:hidden"
       on:click={handleHamburgerClick}
-      aria-label="Toggle menu"
+      aria-label={$t.common?.toggle_menu || "Toggle menu"}
     >
       <Icon name="menu" size={22} />
     </button>
@@ -136,6 +137,8 @@
 
   <!-- Nav Actions -->
   <div class="flex items-center gap-3 md:gap-4">
+    <LanguageSwitcher />
+
     <!-- Assistant -->
     <button
       class="rounded-lg p-2 text-slate-600 transition-colors hover:bg-slate-100"
@@ -154,7 +157,7 @@
         (e.key === "Enter" || e.key === " ") && handleActivityClick()}
       role="button"
       tabindex="0"
-      aria-label="Activity"
+      aria-label={$t.common?.activity || "Activity"}
     >
       <Icon name="activity" size={22} />
       {#if activeCount > 0}
@@ -174,7 +177,7 @@
           (e.key === "Enter" || e.key === " ") && toggleUserMenu(e)}
         role="button"
         tabindex="0"
-        aria-label="User menu"
+        aria-label={$t.common?.user_menu || "User menu"}
       >
         {#if user}
           <span
@@ -192,7 +195,7 @@
           : 'hidden'}"
       >
         <div class="px-4 py-2 text-sm text-gray-700">
-          <strong>{user?.username || "User"}</strong>
+          <strong>{user?.username || ($t.common?.user || "User")}</strong>
         </div>
         <div class="border-t border-gray-200 my-1"></div>
         <div