refactor: retire tui.BuiltinThemes in favor of theming.List

Make internal/theming.List the single source of truth for themes:

- Drop tui.BuiltinThemes, tui.ThemeByID, and the NvChadTheme field on
  tui.Theme. tui.Theme is now a thin {ID, Name} display value.
- Wizard populates its dropdown from theming.List() and uses
  DisplayName for the label.
- neovim.resolvedTheme uses theming.Get (with theming.List()[0] as a
  fallback) instead of a caller-supplied []tui.Theme slice.
- neovim.New signature loses the themes []tui.Theme parameter; update
  cmd/archduino/main.go and all neovim tests to match.

Break the resulting neovim→theming import cycle (theming previously
imported neovim for WriteChadrc) by inverting the dependency:
theming exposes a ChadrcWriter function hook, and neovim registers
its WriteChadrc into it via package init. The theming test stubs
ChadrcWriter directly to exercise the chadrc-rewrite branch without
pulling neovim in and re-creating the cycle.

Delete the now-obsolete TestBuiltinThemesCarryNvChadName / TestThemeByID*
tests in internal/tui/state_test.go — their coverage lives in the
theming package's own tests.

internal/tui/wizard/wizard.go

@@ -10,6 +10,7 @@ import (
 	"github.com/charmbracelet/huh"
 
 	"github.com/MDietermann/ArduinoArchSetup/internal/module"
+	"github.com/MDietermann/ArduinoArchSetup/internal/theming"
 	"github.com/MDietermann/ArduinoArchSetup/internal/tui"
 )
 
@@ -23,9 +24,10 @@ func Run(ctx context.Context, reg *module.Registry, initial tui.WizardState) (tu
 		return s, false, nil
 	}
 
-	themeOptions := make([]huh.Option[string], 0, len(tui.BuiltinThemes))
-	for _, t := range tui.BuiltinThemes {
-		themeOptions = append(themeOptions, huh.NewOption(t.Name, t.ID))
+	themes := theming.List()
+	themeOptions := make([]huh.Option[string], 0, len(themes))
+	for _, t := range themes {
+		themeOptions = append(themeOptions, huh.NewOption(t.DisplayName, t.ID))
 	}
 
 	moduleOptions := make([]huh.Option[string], 0, len(s.Modules))