Add VSCode-style scrollbar change indicators to diff viewer #2315

Summary

Add a minimap/scrollbar gutter indicator (like VSCode) to the diff viewer so users can quickly see where changes are located within a file without scrolling through the entire diff.

Context

In VSCode, the right-side scrollbar shows colored markers (green for additions, red for deletions, blue for modifications) that give an at-a-glance overview of where changes exist in a file. This is especially useful for large files where changes are spread across different sections.

Our current diff viewer (LightDiffViewer using @pierre/diffs/react) shows the diff content itself well, but lacks this birds-eye-view navigation aid.

Proposed Behavior

• Show a narrow gutter/track alongside the scrollbar (or overlaid on it) with colored indicators:
  • Green markers for added lines
  • Red markers for deleted lines
  • Yellow/Blue markers for modified lines
• Markers should be proportionally positioned to reflect where changes sit in the file
• Clicking a marker should scroll to that change
• Should work in both split and unified diff views
• Use existing theme color tokens (--diff-add-bg, --diff-remove-bg, etc.) for consistency

Relevant Files

• apps/desktop/src/renderer/screens/main/components/WorkspaceView/ChangesContent/components/LightDiffViewer/LightDiffViewer.tsx
• apps/desktop/src/renderer/screens/main/components/WorkspaceView/ChangesContent/components/FileDiffSection/FileDiffSection.tsx
• apps/desktop/src/renderer/screens/main/components/WorkspaceView/utils/code-theme.ts

Reference

VSCode's scrollbar decorations: the colored tick marks in the scrollbar track that indicate additions, deletions, modifications, search matches, errors, etc.