Improve preview and partial upload of changed Gist files

[landeqiming666]

Summary

• build one change plan from the local and remote Gist file sets
• show the exact created, changed, and deleted files before a normal upload
• send a minimal Gist PATCH, including explicit deletions while preserving cross-platform keybindings* files
• retain the existing full-payload behavior for forced uploads

This covers the preview/confirmation path in addition to the minimal-patch and deletion semantics already explored in the other proposals for #508.

Validation

• npm run tslint-check
• TS_NODE_TRANSPILE_ONLY=1 node -r ts-node/register ./node_modules/mocha/bin/mocha test/service/gistChange/gistChange.test.ts --reporter spec (4 passing)
• git diff --check

The repository has no lockfile. On a current Node installation, its full TypeScript compilation resolves incompatible transitive type packages and fails inside existing @octokit/rest declarations before checking this change. The focused behavioral tests and lint pass without modifying project dependencies.

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IssueHunt Summary

Referenced issues

This pull request has been submitted to:

• #508: Compare Local & Gist Settings | Show & upload only changes

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[landeqiming666]

Hi, a brief follow-up on this PR. It remains mergeable and keeps the change scoped to previewing changed Gist files and uploading only the selected changes. No code-check failure is currently shown. Happy to add coverage or narrow the UI flow further.