feat: add pagination and detail pages for commits and PRs

Related Issue

N/A

Changes

• Add page-based pagination to commit history (50/page) and PR list (30/page)
• Add PR detail page with Overview (Markdown rendered), Commits, and Files changed tabs
• Add commit detail page with file diff view
• Add FilesChanged client component with per-file and global fold/unfold toggle
• Add MarkdownBody component using react-markdown + remark-gfm
• Add GitHub API helpers: getCommit, getPull, getPullFiles, getPullCommits
• Update i18n strings (ko/en) for all new UI text

How to Verify

1. Navigate to the Commits tab — verify prev/next pagination buttons appear correctly
2. Click a commit — verify detail page shows commit message and colored diff
3. Navigate to the PR list — verify pagination works
4. Click a PR — verify Overview tab renders Markdown (headings, lists, code blocks, tables, task lists)
5. Click Commits tab in PR detail — verify list appears and each item links to commit detail
6. Click Files changed tab — verify diff is shown with + green / - red coloring
7. On Files changed, test fold/unfold per file and global toggle button
8. Switch locale (ko/en) — verify all new strings display correctly
9. Toggle dark mode — verify all new pages render correctly

Checklist

• npm run build passes
• npm run lint passes
• New/updated strings added to both locales/ko.json and locales/en.json (if applicable)