feat: Implement core frontend application with task management, user selection, and a collapsible sidebar.

Provider/frontend/src/lib/UserManager.svelte

@@ -59,9 +59,13 @@
   <div class="user-bar">
     <div class="user-chips">
       {#each users as user}
-        <button
+        <!-- svelte-ignore node_invalid_placement_ssr -->
+        <div
           class="user-chip {selectedUserId === user.id ? 'selected' : ''}"
           onclick={() => selectedUserId = user.id}
+          role="button"
+          tabindex="0"
+          onkeydown={(e) => { if (e.key === 'Enter') selectedUserId = user.id; }}
         >
           <span class="user-name">{user.name}</span>
           {#if confirmDeleteId === user.id}
@@ -76,11 +80,12 @@
               title="Remove user"
             >✕</button>
           {/if}
-        </button>
+        </div>
       {/each}
 
       {#if showAddForm}
         <form class="add-user-form" onsubmit={(e) => { e.preventDefault(); handleAddUser(); }}>
+          <!-- svelte-ignore a11y_autofocus -->
           <input
             type="text"
             bind:value={newUserName}