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/backend/services/task_manager.py

@@ -28,9 +28,10 @@ class Task:
     result_files: List[str] = field(default_factory=list)
     error: Optional[str] = None
     log_lines: List[str] = field(default_factory=list)
+    metadata: Optional[dict] = None
 
     def to_dict(self) -> dict:
-        return {
+        d = {
             "task_id": self.id,
             "name": self.name,
             "status": self.status.value,
@@ -40,6 +41,9 @@ class Task:
             "error": self.error,
             "log_lines": self.log_lines[-100:],
         }
+        if self.metadata:
+            d["metadata"] = self.metadata
+        return d
 
 
 class TaskManager:
@@ -135,6 +139,21 @@ class TaskManager:
             )
         self._schedule(self._broadcast(task_id))
 
+    def retry_task(self, task_id: str) -> Optional[Task]:
+        """Create a new task to retry a failed task with its original parameters.
+
+        Returns the new task if the original was failed and retryable, else None.
+        The caller is responsible for dispatching the actual work based on
+        ``new_task.metadata``.
+        """
+        original = self._tasks.get(task_id)
+        if not original or original.status != TaskStatus.FAILED:
+            return None
+        new_task = self.create_task(original.name)
+        if original.metadata:
+            new_task.metadata = dict(original.metadata)
+        return new_task
+
     def set_failed(self, task_id: str, error: str):
         task = self._tasks.get(task_id)
         if not task: