fix: change DOING takeover behavior to PENDING and enforce status sort order

- When setting a new task to DOING, the previous DOING task now switches to PENDING instead of TODO
- Task display order now prioritizes by status: DONE < DOING < PENDING < TODO, with order field respected within each status group

app.py

@@ -347,7 +347,7 @@ def api_update_task_status(task_id):
                 doing_task = t
                 break
         if doing_task:
-            database.update_task(doing_task["id"], status="todo")
+            database.update_task(doing_task["id"], status="pending")
         updates["start_time"] = datetime.now().isoformat()
     else:
         updates["start_time"] = None

database.py

@@ -209,7 +209,12 @@ def get_tasks_sorted(goal_id):
 
         cur = conn.execute(
             """SELECT * FROM tasks WHERE goal_id = ? AND status != 'done'
-               ORDER BY "order" ASC""",
+               ORDER BY CASE status
+                   WHEN 'doing' THEN 1
+                   WHEN 'pending' THEN 2
+                   WHEN 'todo' THEN 3
+                   ELSE 4
+               END, "order" ASC""",
             (goal_id,)
         )
         unfinished = [row_to_dict(r) for r in cur.fetchall()]