Diff: Add inline (intra-line) change highlighting

[vipmax]

Problem

The current diff view highlights only whole added/removed lines.
This makes it hard to quickly understand what changed inside a modified line.

Current behavior

Whole lines are colored, but changed fragments inside a line are not:

Expected behavior

In modified lines, changed fragments should be highlighted inline (word/segment level), similar to GitHub-style diffs:

• removed fragments with a red inline background,
• added fragments with a green inline background.

Scope

Add inline change highlighting for modified lines while keeping existing line-level highlighting.

Acceptance Criteria

• Inline highlighting is shown for modified lines (not only whole-line add/remove background).
• Highlighting works correctly for TypeScript code and does not break current syntax highlighting.
• No noticeable performance regression on large diffs.
• Tests are added/updated for diff rendering logic (where applicable).

Implementation Notes

A word/token-level diff algorithm can be used (for example, diff-match-patch or equivalent), then rendered as inline spans on top of existing diff markup.