fix: chrome extension with Brave flatpack

[kdihalas]

This pull request adds support for Brave Browser installed as a Flatpak on Linux, ensuring that native messaging works correctly in sandboxed environments. It introduces a Flatpak-aware wrapper script for Brave Flatpak, updates the manifest generation logic, and thoroughly tests the new behavior.

Changes

• Added a new entry for "Brave (Flatpak)" to the list of native messaging browser targets on Linux, including a dedicated wrapper script path (pearpass-wrapper.sh).
• Implemented buildFlatpakBrowserWrapperContent in electron/nativeHostWrapper.cjs to generate a Flatpak-aware wrapper script, handling both regular and PearPass Flatpak cases.
• Updated native messaging setup to write the Flatpak wrapper script, set executable permissions, and point the Brave Flatpak manifest to the wrapper.
• Modified the manifest and cleanup logic to account for the new Brave Flatpak entry. [1] [2]

Testing Notes

• Added comprehensive tests for the new wrapper logic and updated all relevant test cases to reflect the additional Brave Flatpak target, including manifest counts and file writes. [1] [2] [3] [4] [5]

• Run a functional test on Linux with Brave Flatpack

These changes ensure that PearPass integrates seamlessly with Brave Flatpak, maintaining robust native messaging support across more Linux environments.