Extract isomorphic @getpaseo/highlight package with syntax-highlighted file preview

- Create packages/highlight with shared Lezer-based syntax highlighting (types, parsers, highlighter, color maps)
- Add syntax highlighting and line numbers to file-pane.tsx with memoized CodeLine component
- Import shared highlight colors in git-diff-pane.tsx instead of local definitions
- Delete duplicated syntax-highlighter.ts from both app and server
- Fix TSX dialect to "ts jsx" (correct per Lezer docs)

packages/highlight/src/index.ts

@@ -0,0 +1,4 @@
+export type { HighlightStyle, HighlightToken } from './types.js'
+export { getParserForFile, isLanguageSupported, getSupportedExtensions } from './parsers.js'
+export { highlightCode, highlightLine } from './highlighter.js'
+export { darkHighlightColors, lightHighlightColors } from './colors.js'