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/colors.ts

@@ -0,0 +1,47 @@
+import type { HighlightStyle } from './types.js'
+
+export const darkHighlightColors: Record<HighlightStyle, string> = {
+  keyword: '#ff7b72',
+  comment: '#8b949e',
+  string: '#a5d6ff',
+  number: '#79c0ff',
+  literal: '#79c0ff',
+  function: '#d2a8ff',
+  definition: '#d2a8ff',
+  class: '#ffa657',
+  type: '#ff7b72',
+  tag: '#7ee787',
+  attribute: '#79c0ff',
+  property: '#79c0ff',
+  variable: '#c9d1d9',
+  operator: '#79c0ff',
+  punctuation: '#c9d1d9',
+  regexp: '#a5d6ff',
+  escape: '#79c0ff',
+  meta: '#8b949e',
+  heading: '#79c0ff',
+  link: '#a5d6ff',
+}
+
+export const lightHighlightColors: Record<HighlightStyle, string> = {
+  keyword: '#cf222e',
+  comment: '#6e7781',
+  string: '#0a3069',
+  number: '#0550ae',
+  literal: '#0550ae',
+  function: '#8250df',
+  definition: '#8250df',
+  class: '#953800',
+  type: '#cf222e',
+  tag: '#116329',
+  attribute: '#0550ae',
+  property: '#0550ae',
+  variable: '#24292f',
+  operator: '#0550ae',
+  punctuation: '#24292f',
+  regexp: '#0a3069',
+  escape: '#0550ae',
+  meta: '#6e7781',
+  heading: '#0550ae',
+  link: '#0a3069',
+}