* Improve postinstall message to detect existing Chrome installations
Previously, the npm postinstall script always recommended running `agent-browser install` to download Chrome for Testing, even when users already had a working Chrome installation on their system.
This change adds Chrome detection logic to the postinstall script that mirrors the runtime behavior:
- Checks for system Chrome installations on macOS, Linux, and Windows
- Shows a success message when Chrome is found, indicating it will be used automatically
- Only shows the `agent-browser install` warning when no Chrome is detected
- Provides platform-specific guidance (Linux `--with-deps` flag, `--executable-path` alternative)
The detection logic matches the existing Rust `find_chrome()` implementation to ensure consistency between postinstall messaging and runtime behavior.
Fixes#814
* Mention --cdp, --provider, --engine as alternatives in postinstall message
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Co-authored-by: ctate <366502+ctate@users.noreply.github.com>
