Fix lists indentation issues

[Twinsen81]

Closes DODROID-433

Description

• Fixes the indentation issue for a task list (1435aec)
• Fixes a rendering issue where outdenting an empty list item (no text) removes the visible marker (bullet/number/checkbox) even though the item stays a list internally (150484e)
• Adds the task list and indentation buttons to the demo app's toolbar (9c82adb)
• Fixes the build issue of the sample app (Java 21 compatibility) (187e981)

Screen_recording_20251022_125151.mp4

Test

• Check out the branch dodroid-433-the-editor-fails-to-add-checklist-indentation
• Build and run the sample app
• Create an unordered list with at least two items
• Add a new empty list item (press Enter)
• Press outdent on the empty item
• Verify the bullet remains visible immediately after outdent (no typing required)
• Repeat the steps above for an ordered list
• Verify numbering shows correctly after outdent
• Create a task list with at least two items
• Press indent on an item
• Verify indentation works correctly
• Add more items and indent them further - verify it works correctly
• Add a new empty list item (press Enter)
• Press outdent on the empty item
• Verify the checkbox remains visible immediately after outdent (no typing required)

Screen_recording_20251022_124823.mp4

[Twinsen81]

@mzorz Just a quick nudge to take a look at this PR when you get a chance.

[mzorz]

LGTM! works as described