feat: processing flow enhancement + responsive UI

Phase 2 - Processing flow:
- Multi-task monitoring: store supports concurrent task tracking
- Task retry: POST /api/tasks/{id}/retry re-runs failed tasks
- Dashboard multi-task cards with progress, error details, retry/dismiss
- Log panel expanded from 10 to 50 lines with "view all" link

Phase 3 - UI/UX:
- Mobile sidebar drawer (< 768px) with hamburger menu
- Layout responsive styles (768px, 480px breakpoints)
- Tasks/Logs pages responsive (stat cards, filters, columns)
- File views responsive (header wrap, button sizing)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/frontend/src/views/Tasks.vue

@@ -453,4 +453,35 @@ onUnmounted(cancelSearch)
 
 .log-line.err { color: #f87171; }
 .log-line.ok { color: #34d399; }
+
+/* ── Responsive ── */
+@media (max-width: 768px) {
+  .stats-row {
+    grid-template-columns: repeat(2, 1fr);
+    gap: 8px;
+  }
+  .stat-card {
+    padding: 14px 16px;
+  }
+  .stat-value {
+    font-size: 20px;
+  }
+  .card-head {
+    flex-direction: column;
+    align-items: flex-start;
+    gap: 12px;
+  }
+  .card-actions {
+    flex-wrap: wrap;
+    gap: 6px;
+  }
+  .card {
+    padding: 14px;
+  }
+}
+@media (max-width: 640px) {
+  .stats-row {
+    grid-template-columns: 1fr;
+  }
+}
 </style>