Fix checkbox interaction for indented (nested) task list items

[zwarm]

Fix

Fix checkbox interaction for indented (nested) task list items.

Note

This PR can be tested with DayOne-Android via the companion PR here

Three issues were addressed:

1. Cannot check indented items: The x-coordinate gate in AztecText.onTouchEvent() and TaskListClickHandler only allowed taps within a single leading margin width, rejecting taps on nested checkboxes positioned further right. The handler now computes the tap boundary based on the nesting depth of the tapped item.
2. Checking item above indented item checks both: isChecked() in AztecTaskListSpan fetched all AztecListItemSpan children (including nested ones) but used a line index that only counted direct children, causing an index mismatch. Now filters spans by nestingLevel to match.
3. Wrong item gets strikethrough: Same root cause as TagSoup fixes HTML when there is no root node #2 — after any nested item, every subsequent checkbox rendered the state of the wrong item.

Test Steps

1. Check out the branch dodroid-954-checklist-issue
2. Build and run the sample app
3. Create a task list with a few items
4. Indent one item in the middle using the indent toolbar button
5. Tap the checkbox on the indented item — it should toggle independently
6. Tap the checkbox on the item directly above the indented item — only that item should be checked
7. Tap the checkbox on the item directly below the indented item — the correct item should be checked and struck through
8. Verify items at all positions (above, below, indented) can be checked and unchecked independently

Make sure strings will be translated:

• If there are new strings that have to be translated, I have added them to the client's strings.xml as a part of the integration PR.