fix(server): serve cached index.html in SPA catch-all to prevent 500

res.sendFile can emit NotFoundError from the send module in certain
path resolution scenarios, causing 500s on company-scoped SPA routes.
Cache index.html at startup and serve it directly, which is both
more reliable and faster.

Fixes #233

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/app.ts

@@ -134,9 +134,10 @@ export async function createApp(
     ];
     const uiDist = candidates.find((p) => fs.existsSync(path.join(p, "index.html")));
     if (uiDist) {
+      const indexHtml = fs.readFileSync(path.join(uiDist, "index.html"), "utf-8");
       app.use(express.static(uiDist));
       app.get(/.*/, (_req, res) => {
-        res.sendFile("index.html", { root: uiDist });
+        res.status(200).set("Content-Type", "text/html").end(indexHtml);
       });
     } else {
       console.warn("[paperclip] UI dist not found; running in API-only mode");