feat: warn user when unsupported 'delta --navigate' flag is used

[bit2swaz]

Closes #5136

The Issue
Users attempting to use delta --navigate were finding that navigation keys (n/N) were unresponsive. This is because lazygit reads the pager's stdout via a PTY but does not forward stdin to the subprocess, making interactive features architecturally impossible in the current view.

The Fix
I added a startup check in onInitialViewsCreation (in pkg/gui/layout.go). If the user's config contains delta and --navigate, a modal dialog now appears explaining that this feature is not supported and advising them to remove the flag. The warning is also logged to the debug log for reference.

Why This Happens
Lazygit captures pager output via a PTY to display it in the TUI, but keyboard input goes through lazygit's own keybinding system rather than being forwarded to the subprocess. This architectural design means interactive pager features cannot work as intended.

Validation
I reproduced the issue with a local config and verified the dialog appears on startup:

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

[stefanhaller]

I don't think we need to check for this and add a warning. It might conflict with other panels that we sometimes show at startup, and it's not worth dealing with the complexities that this would bring.

Also, I'm honestly surprised that anybody expects this to work in the first place.

I opened #5155 to add a note about this to the documentation, I think this is all we need to do.