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/tsconfig.json

@@ -0,0 +1,20 @@
+{
+  "compilerOptions": {
+    "target": "ES2020",
+    "module": "NodeNext",
+    "moduleResolution": "NodeNext",
+    "lib": ["ES2020"],
+    "strict": true,
+    "skipLibCheck": true,
+    "isolatedModules": true,
+    "esModuleInterop": true,
+    "allowSyntheticDefaultImports": true,
+    "declaration": true,
+    "declarationMap": true,
+    "sourceMap": true,
+    "outDir": "./dist",
+    "rootDir": "./src"
+  },
+  "include": ["src/**/*"],
+  "exclude": ["node_modules", "dist", "src/**/*.test.ts"]
+}