Update GitHub Actions workflow triggers to use the master branch
for push and pull_request events. Replace the previous nx branch
restriction so CI (build, make and e2e tests) runs on master and tag
events as intended.
This enables workflows to execute for changes merged to master and
aligns branch configuration across workflows.
Add a new GitHub Actions workflow "Cross-Platform E2E Tests" that runs
end-to-end tests on linux, macOS, and Windows using a matrix job.
The workflow checks out code, sets up Node.js 22 with npm cache,
installs deps, builds the backend, installs Playwright browsers, and
runs the electron-backend-e2e suite (using xvfb on Linux). Failed
test artifacts (Playwright report) are uploaded per OS for 7 days.
Remove the previous monolithic 'Build and Test' workflow which included
unit tests, coverage upload, and an older E2E configuration. This
consolidates E2E across platforms into a focused, cross-platform job
and simplifies the CI surface by deprecating the outdated workflow.
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