feat: show diff line count (+/-) for changed files

[ClaudiaFang]

Background

When syncing files, the plugin currently shows which files changed, but not how much each file changed.

Goal

Display the number of added/removed lines for each changed file, similar to a standard git diff stat (e.g. +12 -4), so users can quickly gauge the size of a change before syncing/pulling.

Open Questions

• Where should this be shown — file list in the sync modal, status bar, or both?
• Should it be a simple +X -Y count, or a full diff view on demand (e.g. click to expand)?

[ClaudiaFang]

Backlog priority review — 2026-08-04

Priority: P3

Reason: The +N/−M diff summary is already explicitly included in #86’s DiffPanel improvement scope. Keeping a separate active feature creates duplicate implementation and acceptance tracking.

Scheduling note: Fold the desired display location and behavior into #86, then close this issue as duplicate unless a later requirement specifically targets per-file list statistics outside the diff panel.