Compare local vs remote Gist before upload | Show diff preview (#508)

[yanrongfei]

Summary

Fixes #508 — Compare local and remote Gist settings before upload; show diff preview and only upload changed files.

Changes

• Before uploading: fetch the remote Gist and compare each local file's content with the remote Gist file
• Diff preview: show a dedicated Output Channel listing changed files (with local/remote line counts) and unchanged files
• User confirmation: display a dialog "Code Settings Sync: Changes detected. Do you want to upload?" with Upload/Cancel buttons
• Summary output: shows only the files that actually changed (not all files)
• cloudSettings is always treated as changed (its timestamp is updated on every upload)
• Graceful fallback: on first upload (no remote Gist yet) or errors, all files are still uploaded as before

Testing

1. Make a small change to one settings file (e.g. settings.json)
2. Run the upload command
3. Verify the diff preview shows only the changed file
4. Confirm the upload and check the summary matches