Add git config menu with scope switching

[postgnostic]

PR Description

Adds a Git config menu (global keybinding ;) for viewing and editing git config values in a tree view. Supports
switching scope (local/global/system) via l/g/s or ←/→, shows the active scope in the header, persists expanded
sections, and includes common keys even when unset.

Notes

• Future idea (after approval): when committing without user.name/user.email, prompt to open this menu.
• Observation: Integration tests need global user.name/user.email; I prefer per‑repo settings. It’d be nice
  if tests could set these locally or reuse the repo’s local config.

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

[postgnostic]

ok should be ready now