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

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

@@ -201,7 +201,7 @@
         <span class="inline-flex h-6 w-6 items-center justify-center rounded-md bg-gradient-to-br from-slate-100 to-slate-200 text-slate-700 ring-1 ring-slate-200">
           <Icon name="layers" size={14} />
         </span>
-        Menu
+        {$t.nav?.menu || "Menu"}
       </span>
     {:else}
       <span class="text-xs text-gray-500">M</span>
@@ -285,7 +285,7 @@
         <span class="mr-2 inline-flex h-6 w-6 items-center justify-center rounded-md bg-slate-100 text-slate-600">
           <Icon name="chevronLeft" size={14} />
         </span>
-        Collapse
+        {$t.nav?.collapse || "Collapse"}
       </button>
     </div>
   {:else}
@@ -293,10 +293,10 @@
       <button
         class="flex items-center justify-center w-full px-4 py-2 text-sm text-gray-600 hover:bg-gray-100 rounded-lg transition-colors"
         on:click={handleToggleClick}
-        aria-label="Expand sidebar"
+        aria-label={$t.nav?.expand_sidebar || "Expand sidebar"}
       >
         <Icon name="chevronRight" size={16} />
-        <span class="ml-2">Expand</span>
+        <span class="ml-2">{$t.nav?.expand || "Expand"}</span>
       </button>
     </div>
   {/if}