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/server/package.json

@@ -64,22 +64,8 @@
     "@ai-sdk/openai": "2.0.52",
     "@anthropic-ai/claude-agent-sdk": "^0.2.11",
     "@deepgram/sdk": "^3.4.0",
+    "@getpaseo/highlight": "*",
     "@getpaseo/relay": "0.1.32",
-    "@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",
     "@modelcontextprotocol/sdk": "^1.20.1",
     "@opencode-ai/sdk": "1.2.6",
     "@sctg/sentencepiece-js": "^1.1.0",
@@ -90,7 +76,6 @@
     "express": "^4.18.2",
     "express-basic-auth": "^1.2.1",
     "fast-uri": "^3.1.0",
-    "lezer-elixir": "^1.1.2",
     "mnemonic-id": "^3.2.7",
     "node-pty": "1.2.0-beta.11",
     "onnxruntime-node": "^1.23.0",