fix(browser): guard Browser.create against undefined launch options (#117)

[linhongyu510]

Summary

Fixes #117.

Browser.create() is typed as create(options: { launchOrConnect: LaunchOptions }) and forwards options.launchOrConnect to the private #init. Two paths lead to #init(undefined), after which #processOptions dereferences options.defaultViewport and throws:

TypeError: Cannot read properties of undefined (reading 'defaultViewport')
    at Browser.#processOptions (packages/browser/src/browser/browser.ts:149)
    at Browser.#init (.../browser.ts:75)
    at Browser.create (.../browser.ts:27)

1. A caller passes launch options at the top level, e.g. Browser.create({ headless: false }). This is exactly what the bundled packages/browser/example/browser.ts did, so npx tsx example/browser.ts crashes out of the box (the repro in [Bug]: example/browser.ts fails due to accessing a property of undefined #117).
2. A caller omits launchOrConnect entirely.

Changes

• src/browser/browser.ts: default #init param to {} and use options.launchOrConnect ?? {} inside create, so a missing/undefined launch-options object falls back to defaults instead of crashing.
• example/browser.ts: nest headless under the documented launchOrConnect key so the example matches the public create signature and runs unmodified.

Verification

• Traced the crash to #processOptions reading options.defaultViewport on an undefined argument; both entry paths (create forwarding undefined, and #init receiving no value) are now guarded.
• The example now conforms to the { launchOrConnect: LaunchOptions } signature, matching README quick-start usage (Browser.create() with no args also remains valid).

Note: #119 previously targeted this issue but was closed by its author without merging and only edited the example (also flipping headless to true); this PR fixes the underlying SDK crash as well so the defect cannot recur from other callers.