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/modules/neovim/neovim_test.go

@@ -10,7 +10,6 @@ import (
 	"github.com/MDietermann/ArduinoArchSetup/internal/config"
 	"github.com/MDietermann/ArduinoArchSetup/internal/module"
 	"github.com/MDietermann/ArduinoArchSetup/internal/platform"
-	"github.com/MDietermann/ArduinoArchSetup/internal/tui"
 )
 
 type fakeRunner struct {
@@ -58,7 +57,7 @@ func TestDetectFreshSystem(t *testing.T) {
 	home := t.TempDir()
 	t.Setenv("HOME", home)
 	r := &fakeRunner{installed: map[string]bool{}}
-	m := New(config.NeovimConfig{NvChadStarterRepo: "https://x/starter"}, "gruvbox", tui.BuiltinThemes, platform.NewPacman(r), platform.GitClient{Runner: r})
+	m := New(config.NeovimConfig{NvChadStarterRepo: "https://x/starter"}, "gruvbox", platform.NewPacman(r), platform.GitClient{Runner: r})
 	st, err := m.Detect(context.Background())
 	if err != nil {
 		t.Fatal(err)
@@ -81,7 +80,7 @@ func TestDetectNonNvChadTriggersBackup(t *testing.T) {
 	os.MkdirAll(filepath.Join(home, ".config", "nvim", "lua"), 0o755)
 	os.WriteFile(filepath.Join(home, ".config", "nvim", "lua", "plugins.lua"), []byte("-- lazyvim"), 0o644)
 	r := &fakeRunner{installed: map[string]bool{}}
-	m := New(config.NeovimConfig{NvChadStarterRepo: "https://x/starter"}, "gruvbox", tui.BuiltinThemes, platform.NewPacman(r), platform.GitClient{Runner: r})
+	m := New(config.NeovimConfig{NvChadStarterRepo: "https://x/starter"}, "gruvbox", platform.NewPacman(r), platform.GitClient{Runner: r})
 	st, _ := m.Detect(context.Background())
 	if !st.(state).BackupNeeded {
 		t.Fatal("expected BackupNeeded=true")
@@ -92,7 +91,7 @@ func TestPlanApplyInstallsEverything(t *testing.T) {
 	home := t.TempDir()
 	t.Setenv("HOME", home)
 	r := &fakeRunner{installed: map[string]bool{}}
-	m := New(config.NeovimConfig{NvChadStarterRepo: "https://x/starter"}, "dracula", tui.BuiltinThemes, platform.NewPacman(r), platform.GitClient{Runner: r})
+	m := New(config.NeovimConfig{NvChadStarterRepo: "https://x/starter"}, "dracula", platform.NewPacman(r), platform.GitClient{Runner: r})
 	st, _ := m.Detect(context.Background())
 	p, _ := m.Plan(context.Background(), st)
 	if len(p.Actions) == 0 {
@@ -150,7 +149,7 @@ func TestPlanAfterApplyIsEmpty(t *testing.T) {
 	home := t.TempDir()
 	t.Setenv("HOME", home)
 	r := &fakeRunner{installed: map[string]bool{"neovim": true}}
-	m := New(config.NeovimConfig{NvChadStarterRepo: "https://x/starter"}, "gruvbox", tui.BuiltinThemes, platform.NewPacman(r), platform.GitClient{Runner: r})
+	m := New(config.NeovimConfig{NvChadStarterRepo: "https://x/starter"}, "gruvbox", platform.NewPacman(r), platform.GitClient{Runner: r})
 	st, _ := m.Detect(context.Background())
 	p, _ := m.Plan(context.Background(), st)
 	_ = m.Apply(context.Background(), p, nopReporter{})
@@ -165,14 +164,16 @@ func TestUnknownThemeFallsBackToFirst(t *testing.T) {
 	home := t.TempDir()
 	t.Setenv("HOME", home)
 	r := &fakeRunner{installed: map[string]bool{"neovim": true}}
-	m := New(config.NeovimConfig{NvChadStarterRepo: "https://x/starter"}, "does-not-exist", tui.BuiltinThemes, platform.NewPacman(r), platform.GitClient{Runner: r})
+	m := New(config.NeovimConfig{NvChadStarterRepo: "https://x/starter"}, "does-not-exist", platform.NewPacman(r), platform.GitClient{Runner: r})
 	st, _ := m.Detect(context.Background())
 	p, _ := m.Plan(context.Background(), st)
 	if err := m.Apply(context.Background(), p, nopReporter{}); err != nil {
 		t.Fatal(err)
 	}
 	chadrc, _ := os.ReadFile(filepath.Join(home, ".config", "nvim", "lua", "chadrc.lua"))
-	if !strings.Contains(string(chadrc), `theme = "gruvbox"`) {
+	// Unknown theme IDs fall back to the first entry of theming.List(),
+	// which is sorted alphabetically by ID (currently "catppuccin").
+	if !strings.Contains(string(chadrc), `theme = "catppuccin"`) {
 		t.Fatalf("fallback theme not applied: %s", chadrc)
 	}
 }