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_test.go

@@ -6,6 +6,7 @@ import (
 
 	"github.com/MDietermann/ArduinoArchSetup/internal/config"
 	"github.com/MDietermann/ArduinoArchSetup/internal/module"
+	"github.com/MDietermann/ArduinoArchSetup/internal/theming"
 )
 
 type fakeModule struct{ id, name, desc string }
@@ -34,7 +35,7 @@ func TestNewWizardStatePreTicksAllByDefault(t *testing.T) {
 			t.Errorf("%s not selected", m.ID)
 		}
 	}
-	if s.ThemeID != BuiltinThemes[0].ID {
+	if s.ThemeID != theming.List()[0].ID {
 		t.Errorf("theme = %q", s.ThemeID)
 	}
 }
@@ -71,30 +72,3 @@ func TestSelectedIDsOnlyReturnsChecked(t *testing.T) {
 	}
 }
 
-func TestBuiltinThemesCarryNvChadName(t *testing.T) {
-	for _, th := range BuiltinThemes {
-		if th.NvChadTheme == "" {
-			t.Errorf("theme %q has empty NvChadTheme", th.ID)
-		}
-	}
-}
-
-func TestThemeByIDReturnsMatch(t *testing.T) {
-	got, ok := ThemeByID(BuiltinThemes, "dracula")
-	if !ok {
-		t.Fatal("dracula should exist")
-	}
-	if got.ID != "dracula" {
-		t.Fatalf("got id=%q", got.ID)
-	}
-	if got.NvChadTheme == "" {
-		t.Fatal("NvChadTheme empty")
-	}
-}
-
-func TestThemeByIDFallsBackWhenUnknown(t *testing.T) {
-	_, ok := ThemeByID(BuiltinThemes, "does-not-exist")
-	if ok {
-		t.Fatal("unknown id should report ok=false")
-	}
-}