feat: add theme.transparent option for transparent window background

[francescarpi]

Summary

Add a new theme.transparent config flag (default false) that makes the main window and help window render with a transparent background.

Changes

• lua/buffon/config.lua: Add transparent = false to the default theme. When enabled, creates a new BuffonWindow highlight group with bg = NONE, default = true.
• lua/buffon/ui/window.lua: Accept transparent flag in Window:new. When true, applies winhighlight mapping Normal, FloatBorder, FloatTitle, FloatFooter, and EndOfBuffer to BuffonWindow after the window is opened.
• lua/buffon/ui/mainwindow.lua / help.lua: Propagate config.theme.transparent to Window:new.
• tests/config_spec.lua: Updated default config test + 4 new tests covering the transparent flag.
• tests/window_spec.lua (new): 3 tests verifying winhighlight is applied only when transparent = true.
• README.md: New Transparent background section under Theming.

Usage

{
  opts = {
    theme = {
      transparent = true,
    },
  },
}

When enabled, the foreground colors of Buffon's existing highlight groups (shortcut, active, etc.) remain unchanged — only the background becomes transparent. The default false keeps the existing behavior for current users.

Verification

• make lint: 0 warnings, 0 errors
• make test: 49 tests passing (9 new)