fix(studio): add FFmpeg pre-flight check before render (#1100)

* fix(studio): add FFmpeg pre-flight check before starting render

Studio renders now fail fast with a 422 and an actionable FFmpeg
install hint instead of burning through the entire capture pipeline
before hitting "spawn ffmpeg ENOENT" at encode.

* fix(studio): address review — use 503, memoize FFmpeg lookup

packages/cli/src/server/studioServer.ts

@@ -508,6 +508,22 @@ export function createStudioServer(options: StudioServerOptions): StudioServer {
     });
   });
 
+  // ── Pre-flight checks for render ────────────────────────────────────────
+  // Intercept render requests before they reach the shared API so we can
+  // fail fast with an actionable hint instead of burning through the entire
+  // capture pipeline before hitting "spawn ffmpeg ENOENT" at encode.
+  let cachedFFmpegPath: string | undefined;
+  app.post("/api/projects/:id/render", async (c, next) => {
+    const { findFFmpeg, getFFmpegInstallHint } = await import("../browser/ffmpeg.js");
+    if (!cachedFFmpegPath) {
+      cachedFFmpegPath = findFFmpeg();
+    }
+    if (!cachedFFmpegPath) {
+      return c.json({ error: "FFmpeg not found", hint: getFFmpegInstallHint() }, 503);
+    }
+    return next();
+  });
+
   // Mount the shared studio API at /api.
   // Use fetch() forwarding (not .route()) so the sub-app sees paths without
   // the /api prefix — the shared module's path extraction uses c.req.path.