feat: add full-screen break screen overlay

[ArashZich]

Summary

This pull request introduces an opt-in full-screen ambient break overlay ("Break Shield") during break rounds (short-break and long-break), inspired by break screen experiences like GNOME Pomodoro.

When enabled, it gently covers the screen during breaks to encourage stepping away from the monitor, while remaining completely unobtrusive and easy to dismiss if needed.

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What's Changed

• Settings Toggle: Added an opt-in toggle under Settings → Timer (fullscreen_break_shield, defaults to disabled).
• Ambient Break Screen (/break):
  • Full-screen view matching the user's active theme palette and Mona Sans typography.
  • Displays remaining break time, progress ring, round indicator, and subtle ambient breathing glow.
  • Includes mindfulness prompt ("Time to step away and recharge").
• Controls & Interactions:
  • Pause / Resume: Pause or resume the break timer directly from the overlay.
  • Skip Break: Directly advance to the next round if desired.
  • Dismiss / Unlock: Press Esc or click Unlock to close the overlay at any time without stopping or resetting the running break timer.
  • Auto-Dismiss: Automatically closes as soon as the break ends and the next round begins, or when the timer is reset.
• Backend & Database:
  • Added clean SQLite migration (MIGRATION_7) for schema v7 to persist the preference.
  • Declared window capabilities and permissions in Tauri v2 (capabilities/default.json).
• Localization: Added translation strings across all 8 supported languages (en, de, es, fr, ja, pt, tr, zh).
• Changelog: Updated CHANGELOG.md under [Unreleased].

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Verification

• Rust unit tests: cargo test passes cleanly (all 88 tests pass).
• Frontend checks: npm run check passes with 0 errors and 0 warnings.
• Production build: npm run build completes successfully.
• Code style: Prettier / formatting checks verified.
• Manual verification: Built and tested locally on macOS (packaged .dmg). Tested toggling the setting, break transitions, timer pause/resume/skip, and Esc dismissal.