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/parsers.ts

@@ -0,0 +1,78 @@
+import { parser as jsParser } from '@lezer/javascript'
+import { parser as jsonParser } from '@lezer/json'
+import { parser as cssParser } from '@lezer/css'
+import { parser as cppParser } from '@lezer/cpp'
+import { parser as goParser } from '@lezer/go'
+import { parser as htmlParser } from '@lezer/html'
+import { parser as javaParser } from '@lezer/java'
+import { parser as pythonParser } from '@lezer/python'
+import { parser as markdownParser } from '@lezer/markdown'
+import { parser as phpParser } from '@lezer/php'
+import { parser as rustParser } from '@lezer/rust'
+import { parser as xmlParser } from '@lezer/xml'
+import { parser as yamlParser } from '@lezer/yaml'
+import { parser as elixirParser } from 'lezer-elixir'
+import type { Parser } from '@lezer/common'
+
+const parsersByExtension: Record<string, Parser> = {
+  // JavaScript/TypeScript
+  js: jsParser,
+  jsx: jsParser.configure({ dialect: 'jsx' }),
+  ts: jsParser.configure({ dialect: 'ts' }),
+  tsx: jsParser.configure({ dialect: 'ts jsx' }),
+  mjs: jsParser,
+  cjs: jsParser,
+  // C / C++ / Objective-C
+  c: cppParser,
+  h: cppParser,
+  cc: cppParser,
+  cpp: cppParser,
+  cxx: cppParser,
+  hpp: cppParser,
+  hxx: cppParser,
+  m: cppParser,
+  mm: cppParser,
+  // JSON
+  json: jsonParser,
+  // CSS
+  css: cssParser,
+  scss: cssParser,
+  // HTML
+  html: htmlParser,
+  htm: htmlParser,
+  // XML
+  xml: xmlParser,
+  // Java
+  java: javaParser,
+  // Python
+  py: pythonParser,
+  // Go
+  go: goParser,
+  // PHP
+  php: phpParser,
+  // YAML
+  yaml: yamlParser,
+  yml: yamlParser,
+  // Rust
+  rs: rustParser,
+  // Elixir
+  ex: elixirParser,
+  exs: elixirParser,
+  // Markdown
+  md: markdownParser,
+  mdx: markdownParser,
+}
+
+export function getParserForFile(filename: string): Parser | null {
+  const ext = filename.split('.').pop()?.toLowerCase()
+  if (!ext) return null
+  return parsersByExtension[ext] ?? null
+}
+
+export function isLanguageSupported(filename: string): boolean {
+  return getParserForFile(filename) !== null
+}
+
+export function getSupportedExtensions(): string[] {
+  return Object.keys(parsersByExtension)
+}