Add optional staged and unstaged tabs in the files panel

[h4kbas]

PR Description

Add an optional Files-panel tab layout that separates changed files by index state.

When gui.showFileChangeTabs is enabled, the Files window includes two additional tabs:

• Unstaged: shows files with unstaged changes
• Staged: shows files with staged changes

This is integrated into the existing tab system (including [ / ] navigation and tab clicks), while preserving the current default behavior.

By default, this feature is disabled (gui.showFileChangeTabs: false), so existing users keep the current tab layout (Files, Worktrees, Submodules) unless they explicitly opt in.

Also includes a startup safety fix in files-tab indexing/filter application to prevent nil-pointer panics during early initialization.

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