feat: add keybinding to toggle first-parent-only mode in commits panel

[yoyo-67]

Add a keybinding (z) to toggle first-parent-only mode, which shows only commits on the main branch line by passing --first-parent to git log. This provides a cleaner, flatter view for repositories with many merge commits.

Changes:

• Add FirstParentOnly option to GetCommitsOptions
• Add --first-parent flag to git log command
• Add showFirstParentOnly state to LocalCommitsViewModel
• Add z keybinding to toggle the mode
• Add i18n translations

PR Description

Please check if the PR fulfills these requirements

• Cheatsheets are up-to-date (run go generate ./...)
• Code has been formatted (see here)
• Tests have been added/updated (see here for the integration test guide)
• Text is internationalised (see here)
• If a new UserConfig entry was added, make sure it can be hot-reloaded (see here)
• Docs have been updated if necessary
• You've read through your own file changes for silly mistakes etc