[Bug] Infinity scroll for old tasks

**Describe the bug**
After a long period of using the app, the user will have a long list of complete old tasks and opening the app will load all the tasks from storage which is needless and will heavily slow down the app.

**Expected behavior**
There should be infinity scroll loading of tasks. Which means tasks should be loaded in a set of 50 to 100 tasks and the next set will be loaded only when the earlier one is scrolled till bottom.

Start by locating the task-loading entry point and the screen that displays completed tasks; the issue does not name files or tests. Trace how all tasks are loaded from storage, then verify that tasks arrive in batches of 50–100 and that additional batches load only when the list reaches the bottom.

Written by the indexing model from the issue text.