feat: migrate code rendering to @pierre/diffs with JetBrains Mono

Replace @git-diff-view, react-shiki, shiki, and refractor with @pierre/diffs
for unified code rendering across commit diffs, blob viewer, and markdown.

- Commit diffs use MultiFileDiff with pierre-dark theme and built-in file headers
- Blob viewer uses lazy-loaded File component for syntax highlighting
- Extensionless/dotfiles (LICENSE, .gitignore) render with plain text viewer
- Add JetBrains Mono as monospace font via @fontsource-variable
- Remove getLanguageFromFilename and manual Shiki highlighter singleton
- Remove 5 deps (shiki, react-shiki, refractor, @git-diff-view/*), add 2