chore: update version to 1.4.0 and document changes in CHANGELOG

feat(proxy-favicon): disallow SVG favicons and add unit tests for handling

refactor(tests): enhance accessibility tests and improve focus management in UpdateModal

feat(tasks): add detailed task documents for refactoring App.tsx state management and optimizing linkMap updates

feat(tasks): add various task documents for feature enhancements and security improvements

Refactor code structure and remove redundant functions for improved readability and maintainability

refactor: update import paths to use src directory structure

refactor: reorganize project structure into src directory

refactor(preview): remove unnecessary tabIndex from LinkWithPreview component

feat(settings): improve reset settings functionality to clear only app-specific localStorage keys

refactor(ui): enhance layout and accessibility features

feat(ui): suggest trusted users from frequent senders

feat(ui): add help modal and keyboard shortcuts

feat(config): support env-injected app version and default user

ci(config): add GitHub Actions CI workflow