fix: improve cross-device evidence and exploration lifecycle

Align Responses cache affinity across execution and summary calls, preserve required checks and evidence uncertainty, and allow safe deletion of inactive explorations while retaining independent reruns.

Validation: backend suite passed except intentional prompt golden drift, then all 17 transcript tests passed after snapshot refresh; 268 frontend tests and production build passed; Ruff, quality ratchet and core type checks passed.

apps/showcase_ui/src/app/components/chat-interface/chat-interface.component.html

@@ -59,6 +59,7 @@ limitations under the License.
     <!-- Tasks Tab Content -->
     @if (agentService.activeTab() === 'tasks') {
       <app-task-batch-filter />
+      @if (errorMessage()) { <p class="error-message" role="alert">{{ errorMessage() }}</p> }
       <!-- Active Queue Section -->
       <div class="queue-section-header">
         <span class="material-symbols-outlined section-icon">bolt</span> 当前队列 ({{ activeQueue().length }})
@@ -99,7 +100,7 @@ limitations under the License.
                 <span class="task-time-divider">·</span>
               }
               <span class="task-time">{{ session.start_time * 1000 | date:'shortTime' }}</span>
-              @if (getTaskStatus(session) === 'running' || getTaskStatus(session) === 'paused') {
+              @if (!agentService.canDeleteSession(session) && (getTaskStatus(session) === 'running' || getTaskStatus(session) === 'paused')) {
                 <button
                   type="button"
                   class="btn-stop-card"
@@ -110,7 +111,7 @@ limitations under the License.
                   <span class="material-symbols-outlined">stop_circle</span>
                 </button>
               }
-              @if (getTaskStatus(session) !== 'running' && getTaskStatus(session) !== 'paused') {
+              @if (agentService.canDeleteSession(session)) {
                 <button
                   type="button"
                   class="btn-delete-card"