Add a modal dialog to initialize a new Git repository, replacing the
previous prompt-based flow. It integrates with the app state and invokes
the initialization operation then opens the repository on confirmation.
Also adds styles, keyboard handling, accessibility hints, a folder
browser, initial-branch input, and German/English labels with busy state.
- New dialog component with folder picker, branch input, and i18n.
- Wire dialog open/close and confirm handler to init and open actions.
- Add CSS rules for dialog layout, controls, and error/busy states.
