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.go

@@ -17,12 +17,19 @@ 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"
+	"github.com/MDietermann/ArduinoArchSetup/internal/theming"
 )
 
 //go:embed assets/*
 var assets embed.FS
 
+// Register WriteChadrc with the theming package so theming.Apply can
+// rewrite chadrc.lua on theme changes without importing this package
+// (which would be a cycle: neovim imports theming).
+func init() {
+	theming.ChadrcWriter = WriteChadrc
+}
+
 // assetMap lists each asset and its destination under ~/.config/nvim.
 // Order matters: Detect iterates in this order and Apply writes in this order.
 var assetMap = []struct {
@@ -40,14 +47,13 @@ var assetMap = []struct {
 type Module struct {
 	cfg     config.NeovimConfig
 	themeID string
-	themes  []tui.Theme
 	pm      platform.PackageManager
 	git     platform.GitClient
 }
 
 // New constructs a Neovim Module.
-func New(cfg config.NeovimConfig, themeID string, themes []tui.Theme, pm platform.PackageManager, git platform.GitClient) Module {
-	return Module{cfg: cfg, themeID: themeID, themes: themes, pm: pm, git: git}
+func New(cfg config.NeovimConfig, themeID string, pm platform.PackageManager, git platform.GitClient) Module {
+	return Module{cfg: cfg, themeID: themeID, pm: pm, git: git}
 }
 
 type state struct {
@@ -79,11 +85,11 @@ func nvimCfg() (string, error) {
 }
 
 func (m Module) resolvedTheme() string {
-	if t, ok := tui.ThemeByID(m.themes, m.themeID); ok && t.NvChadTheme != "" {
+	if t, ok := theming.Get(m.themeID); ok && t.NvChadTheme != "" {
 		return t.NvChadTheme
 	}
-	if len(m.themes) > 0 {
-		return m.themes[0].NvChadTheme
+	if list := theming.List(); len(list) > 0 && list[0].NvChadTheme != "" {
+		return list[0].NvChadTheme
 	}
 	return "gruvbox"
 }