Add customizable global shortcuts

[luisKisters]

Summary

• add configurable global shortcuts in Settings, between Privacy and Updates
• show or hide the main window with one shortcut
• resume the most recent work entry or stop the running timer with another shortcut
• support Command, Control, Option, and Shift together on macOS
• show a clear error when macOS or another app already uses a shortcut

Screenshots

https://github.com/user-attachments/assets/908d9f25-20da-4a8d-b8f9-96326c2d8106

Tests

• npm run build
• npm run test:unit — 91 passed, 1 environment-specific test skipped
• global shortcut E2E tests — 3 passed
• full E2E run — 46 passed; the existing short-idle test failed locally because the host was already in the macOS idle state before the test changed its threshold

Closes #122
Related to #17

[CLAassistant]

All committers have signed the CLA.

[github-actions]

Hi @luisKisters, thanks for your interest in contributing!

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This PR will be closed automatically. See https://github.com/solidtime-io/solidtime-desktop/blob/main/CONTRIBUTING.md for more details.