Add configurable whitespace replacement for branch names

[SergioChan]

Summary

• add git.branchWhitespaceChar config with default -
• use configured replacement when sanitizing branch names for create/move/rename flows
• keep backward compatibility by falling back to - when config is empty
• add unit tests for branch-name sanitization

Testing

• go test ./pkg/gui/controllers/helpers ./pkg/gui/controllers ./pkg/config

Fixes #2663