fix(picker): correct z-index for preview="main" layout

When using `preview="main"`, the preview window floats over the main
editor window. Set its z-index to 40 (lower than the default 50) to
ensure input and help windows remain on top and visible.

Fixes #2257 (first issue)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

lua/snacks/picker/core/preview.lua

@@ -90,6 +90,7 @@ function M.new(picker)
     main = {
       relative = "win",
       backdrop = false,
+      zindex = 40, -- Lower than default (50) so input/help windows stay on top
     },
     layout = {
       backdrop = win_opts.backdrop == true,