chore: stabilize Playwright E2E runs (#2164)

#1351 made `styleStore` initialization asynchronous in
`componentDidMount`. `setStyle` currently treats `toolbar:link`
appearing as the signal that loading is complete, but if the style
network request (`empty-style.json` from jsdelivr) takes over ~1s, the
`toolbar:link` can appear while `styleStore` is still null (because the
request has not finished). Subsequent `saveStyle` calls in tests then
silently do nothing, causing test failures (3 in #2157, and often 20+ on
my machine).

Changes are:

1. Mock the default empty style to avoid slow network E2E failures.
2. Also cap Playwright workers at 4 to avoid overloading Chromium on
high-core machines.

Fixes #2158.

## Launch Checklist

<!-- Thanks for the PR! Feel free to add or remove items from the
checklist. -->


 - [x] Briefly describe the changes in this PR.
 - [x] Link to related issues.
- [ ] ~~Include before/after visuals or gifs if this PR includes visual
changes.~~
 - [ ] ~~Write tests for all new functionality.~~
- [ ] ~~Add an entry to `CHANGELOG.md` under the `## main` section.~~
(only minor test infra fix)

Co-authored-by: liuly <me@liuly.moe>
Co-authored-by: Harel M <harel.mazor@gmail.com>

e2e/maputnik-driver.ts

@@ -1,5 +1,6 @@
 import { PlaywrightHelper } from "./playwright-helper";
 import { ModalDriver } from "./modal-driver";
+import emptyStyle from "../src/config/empty-style.json" with { type: "json" };
 
 const baseUrl = "http://localhost:8888/";
 const isMac = process.platform === "darwin";
@@ -56,6 +57,11 @@ export class MaputnikDriver {
         url: "https://www.glyph-server.com/*",
         response: ["Font 1", "Font 2", "Font 3"],
       });
+      await this.helper.given.interceptAndMockResponse({
+        method: "GET",
+        url: /cdn\.jsdelivr\.net\/.*\/empty-style\.json$/,
+        response: emptyStyle,
+      });
     },
   };