Allow user to choose specific monitor for break notifications

[N0rvel]

Issue: #1609

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.

• issue was opened to discuss proposed changes before starting implementation. It is important do discuss changes before implementing them (Why should we add it? How should it work? How should it look? Where will it be? ...).
• during development, node version specified in package.json was used (ie using nvm).
• package versions and package-lock.json were not changed (npm install --no-save).
• app version number was not changed.
• all new code has tests to ensure against regressions.
• npm run lint reports no offenses.
• npm run test is error-free.
• README and CHANGELOG were updated accordingly.
• after PR is approved, all commits in it are squashed

Description of the Change

This PR adds a new setting allowing the user to select on which screen/monitor the break window appears.

A new dropdown was added in Preferences > Settings, with options:

• Primary screen
  

• Active Screen (cursor)
  

• Any detected external screen (listed as "Screen 1 — 1920x1080", etc.)
  

The strings are localized and update with language change.

Verification Process

What process did you follow to verify that your change has the desired effects?
Manual testing:

• Changed the setting in the Preferences window, tested with various monitors (including unplug/replug).
• Verified break windows appear on the selected screen or active screen, as intended.
• Switched languages; checked that dropdown entries are properly translated.
• Confirmed that settings persist after app restart.

Regression check:

• Verified that previous screen-related settings (all screens, main screen, etc.) work as before.
  

• Checked that non-modified preferences are unaffected.
  

Other information

I've already used this feature for my personal use for several weeks.

[hovancik]

Hi, you checked issue was opened to discuss proposed changes before starting implementation. It is important do discuss changes before implementing them (Why should we add it? How should it work? How should it look? Where will it be? ...). can you please send me where?

Also, the feature is already there: https://github.com/hovancik/stretchly?tab=readme-ov-file#monitor-to-show-breaks-on-, in advanced settings