build(test): configure Karma browsers from env

[keilogic]

Summary

• Closes Make the protractor browser configurable #551 by letting Karma browser launchers be selected with KARMA_BROWSERS.
• Keeps the current Firefox default when the variable is unset or empty.
• Parses comma-separated browser lists such as ChromeHeadless, FirefoxHeadless.
• Adds policy-test coverage for default, empty, single-browser, multi-browser, and trimmed values.

Validation

• Red baseline before the implementation commit: npm run policy failed with TypeError: karmaConfig.configuredBrowsers is not a function from policy-tests/check-karma-browsers.js.
• npm run policy (passes; output still includes existing historical commit-message warnings)
• $env:KARMA_BROWSERS='FirefoxHeadless'; npx ng test runbox7 --watch=false --include=src/app/storage.service.spec.ts (passes, logs Launching browsers FirefoxHeadless, TOTAL: 11 SUCCESS)
• npx tsc -p src/tsconfig.app.json --noEmit
• npx tsc -p src/tsconfig.spec.json --noEmit
• npm run lint -- --quiet
• git diff --check origin/master...HEAD
• $env:SKIP_CHANGELOG='1'; node src/build/pre-build.js; npx ng build --configuration production --base-href=/app/ runbox7; node src/build/post-build.js (passes, build hash a28fc9fd44996baa)

AI disclosure

This PR was prepared with OpenAI Codex assistance. I used Codex to inspect the issue and current Karma setup, draft the focused policy test and implementation, and run the validation commands listed above.