fix(dsh-plugin-browserskill): prefix routes register without a trailing slash

The webserver prefix matcher joins prefix + '/', so a registered
'/bsk-observation/thumbnail/' never matched real paths.

packages/dsh-plugin-browserskill/src/observation-http.ts

@@ -115,7 +115,7 @@ export function registerObservationRoutes(
     }),
     webServer.register({
       kind: "prefix",
-      path: `${ROUTE_BASE}/thumbnail/`,
+      path: `${ROUTE_BASE}/thumbnail`,
       handler: async (req, res) => {
         if (req.method !== "GET") {
           sendJson(res, 405, { error: "method not allowed" });