Make the focused file visible in the tree

nvim-tree already tracks the current buffer via update_focused_file and
marks it with its own cursorline, but NvimTreeCursorLine links to
CursorLine, which catppuccin sets a shade away from the mantle the tree
draws itself on. The marker was there and unreadable.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

neovim/config/plugins/catppuccin.lua

@@ -7,6 +7,14 @@ require('catppuccin').setup({
     light = 'latte',
     dark = 'macchiato',
   },
+  custom_highlights = function(colors)
+    return {
+      -- CursorLine sits a shade away from the tree's own mantle background,
+      -- so the file you're viewing is invisible in nvim-tree. Give the tree
+      -- a band you can actually pick out.
+      NvimTreeCursorLine = { bg = colors.surface1 },
+    }
+  end,
 })
 
 vim.cmd.colorscheme('catppuccin')