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/state.go

@@ -5,35 +5,15 @@ import (
 
 	"github.com/MDietermann/ArduinoArchSetup/internal/config"
 	"github.com/MDietermann/ArduinoArchSetup/internal/module"
+	"github.com/MDietermann/ArduinoArchSetup/internal/theming"
 )
 
-// Theme is a named colorscheme presented in the wizard. Each entry carries
-// per-tool theme names; Plan 3b uses NvChadTheme, later plans add more.
+// Theme is a named colorscheme presented in the wizard. It is a thin
+// display value for the dropdown; the authoritative theme catalogue lives
+// in the theming package (theming.List / theming.Get).
 type Theme struct {
-	ID          string
-	Name        string
-	NvChadTheme string
-}
-
-// BuiltinThemes is the fixed list the wizard shows. Keep in sync with the
-// themes shipped by Plan 3's theme module.
-var BuiltinThemes = []Theme{
-	{ID: "gruvbox", Name: "Gruvbox Dark", NvChadTheme: "gruvbox"},
-	{ID: "catppuccin", Name: "Catppuccin Mocha", NvChadTheme: "catppuccin"},
-	{ID: "tokyonight", Name: "Tokyo Night", NvChadTheme: "tokyonight"},
-	{ID: "nord", Name: "Nord", NvChadTheme: "nord"},
-	{ID: "dracula", Name: "Dracula", NvChadTheme: "dracula"},
-}
-
-// ThemeByID returns the Theme with the given ID. The bool is false when no
-// match is found; callers decide whether to fall back or error.
-func ThemeByID(themes []Theme, id string) (Theme, bool) {
-	for _, t := range themes {
-		if t.ID == id {
-			return t, true
-		}
-	}
-	return Theme{}, false
+	ID   string
+	Name string
 }
 
 // ModuleChoice is one row in the module selector.
@@ -74,7 +54,10 @@ func NewWizardState(cfg config.Config, reg *module.Registry) WizardState {
 	}
 	sort.Slice(choices, func(i, j int) bool { return choices[i].ID < choices[j].ID })
 
-	theme := BuiltinThemes[0].ID
+	var theme string
+	if list := theming.List(); len(list) > 0 {
+		theme = list[0].ID
+	}
 	if override, ok := cfg.ThemeOverrides["__active"]; ok && override != "" {
 		theme = override
 	}