[FEATURE] Vivid markers for diff changed lines

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Diffs on mobile are illegible to me: I can't tell which lines have been added and removed. This makes it almost impossible to review Claude's work on mobile, and I have to wait until I'm back at my laptop.

<img width="1080" height="1507" alt="screenshot of mobile diff view in dark mode. The background colours vary, but they're nearly indistinguishable." src="https://github.com/user-attachments/assets/989a5705-6a6f-46a8-b40f-811dbd6269b4" />

I'm a bit colour-blind; I can usually tell the difference between red and green but these colours are too desaturated.

Proposed Solution

Option 1: Add a vivid border on the left edge of each changed line.

<img width="1080" height="1507" alt="the same screenshot, but with vivid red and green vertical borders on the left edges of the changed lines" src="https://github.com/user-attachments/assets/a4ba7948-da2c-43e8-9dc6-98001be34617" />

If it was like this screenshot, I could quickly and easily see what's changed.

Option 1b: _Also_ use a unique pattern for each border type. This would be good for people who are very colourblind.

Option 2: Use vivid colours for the line numbers. I do not like this option: even if they were vivid, they would be harder to see than a border.

Option 3: Add + and - markers to the lines. This would be good for people who are very colourblind.

Alternative Solutions

I have to switch to a different app or open my laptop to read the changes. The official GitHub mobile app is actually no better, so don't look to it.

Priority

High - Significant impact on productivity

Feature Category

Other

Use Case Example

_No response_

Additional Context

Trello does option 1b really well with its patterned card labels.

I've called this a feature request but really it's an accessibility bug. IMO it's quite significant.