fix(whiteboard): prevent eraser skipping on fast swipe

[JoelHorrocks]

Purpose / Description

The new study screen whiteboard eraser occasionally skips parts of a drawn path when the user swipes quickly across the screen.

Fixes

• Fixes New Study Screen - Whiteboard Eraser may not erase quick strokes #20555

Approach

• Added eraserLastX and eraserLastY in WhiteboardViewModel to remember the previous eraser coordinates during an erase gesture
• Replaced the check if the current eraser location is close enough to a drawn path with a check if any drawn path is close enough to the last line segment of the eraser stroke

How Has This Been Tested?

Tested on a physical device - drew a line with width 10 on the new whiteboard and tested a quick swipe with eraser width 10 to ensure the line is erased. Also checked that existing eraser functionality works as expected.

Before:

Screen_recording_20260325_135143.mp4

After:

Screen_recording_20260325_135206.mp4

Learning (optional, can help others)

Android MotionEvent touch coordinates can have large gaps between ACTION_MOVE events during fast gestures. These events can batch together coordinates, but even processing the batched coordinates for the eraser, it was still possible for lines to be missed during very fast swipes.

Interpolating between eraser positions by sampling many points along the eraser segment and checking each one against every path was too slow. Instead, projecting each point on the drawn paths onto the eraser line segment to find the shortest distance was much faster.

Checklist

Please, go through these checks before submitting the PR.

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
• UI Changes: You have tested your change using the Google Accessibility Scanner

[welcome]

First PR! 🚀 We sincerely appreciate that you have taken the time to propose a change to AnkiDroid! Please have patience with us as we are all volunteers - we will get to this as soon as possible.

[BrayanDSO]

LGTM. Thanks

[criticalAY]

Clean code! thanks lets get it in