fix(ui): resolve linked highlight groups

[kanna5]

Without the link = false option, vim.api.nvim_get_hl may return linked highlight groups like { link = "GruvboxGreen" }, which are then ignored by create_highlight_group because the foreground and background fields are missing.

Ref: https://neovim.io/doc/user/api.html#nvim_get_hl()