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/package.json

@@ -0,0 +1,36 @@
+{
+  "name": "@getpaseo/highlight",
+  "version": "0.1.32",
+  "private": true,
+  "type": "module",
+  "main": "./dist/index.js",
+  "types": "./src/index.ts",
+  "scripts": {
+    "build": "node -e \"require('node:fs').rmSync('dist',{ recursive: true, force: true })\" && tsc -p tsconfig.json --incremental false",
+    "prepack": "npm run build",
+    "test": "vitest run",
+    "typecheck": "tsc --noEmit"
+  },
+  "dependencies": {
+    "@lezer/common": "^1.5.0",
+    "@lezer/cpp": "^1.1.5",
+    "@lezer/css": "^1.3.0",
+    "@lezer/go": "^1.0.1",
+    "@lezer/highlight": "^1.2.3",
+    "@lezer/html": "^1.3.13",
+    "@lezer/java": "^1.1.3",
+    "@lezer/javascript": "^1.5.4",
+    "@lezer/json": "^1.0.3",
+    "@lezer/markdown": "^1.6.2",
+    "@lezer/php": "^1.0.5",
+    "@lezer/python": "^1.1.18",
+    "@lezer/rust": "^1.0.2",
+    "@lezer/xml": "^1.0.6",
+    "@lezer/yaml": "^1.0.4",
+    "lezer-elixir": "^1.1.2"
+  },
+  "devDependencies": {
+    "typescript": "^5.9.2",
+    "vitest": "^3.2.4"
+  }
+}