chore(theme): re-vendor cms theme copies for font size steps

Picks up FontSizeStepKeys/DefaultFontSizeSteps/ValidateFontSizeSteps and
the FontSizeSteps field on ThemeTypography, so the preset gate's
DisallowUnknownFields parse accepts presets shipping fontSizeSteps.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/theme/defaults.go

@@ -236,6 +236,7 @@ type ThemeTypography struct {
 	LineHeightBase    string            `json:"lineHeightBase"`
 	FontWeightBase    string            `json:"fontWeightBase"`
 	FontSizeOverrides map[string]string `json:"fontSizeOverrides,omitempty"`
+	FontSizeSteps     map[string]string `json:"fontSizeSteps,omitempty"`
 }
 
 // ThemeSpacing represents spacing settings

internal/theme/fontsize.go

@@ -3,6 +3,7 @@ package theme
 import (
 	"fmt"
 	"regexp"
+	"slices"
 	"sort"
 	"strconv"
 	"strings"
@@ -19,6 +20,41 @@ var fontSizeOverrideKeys = map[string]bool{
 
 var fontSizePattern = regexp.MustCompile(`^(\d+(?:\.\d+)?|\.\d+)(rem|px)$`)
 
+var fontSizeStepKeys = []string{"small", "normal", "large", "huge"}
+
+// FontSizeStepKeys returns the canonical step keys in display order.
+func FontSizeStepKeys() []string {
+	return slices.Clone(fontSizeStepKeys)
+}
+
+// DefaultFontSizeSteps returns the fallback rem value for each step.
+func DefaultFontSizeSteps() map[string]string {
+	return map[string]string{
+		"small":  "0.875rem",
+		"normal": "1rem",
+		"large":  "1.125rem",
+		"huge":   "1.25rem",
+	}
+}
+
+// ValidateFontSizeSteps rejects unknown keys and non-rem values. Steps feed the
+// base-size UI chips only and are never emitted to CSS; rem-only so chips always
+// scale with visitor preferences.
+func ValidateFontSizeSteps(steps map[string]string) error {
+	for k, v := range steps {
+		if !slices.Contains(fontSizeStepKeys, k) {
+			return fmt.Errorf("unknown font size step %q (valid: %s)", k, strings.Join(fontSizeStepKeys, ", "))
+		}
+		if err := ValidateFontSize(v); err != nil {
+			return err
+		}
+		if !strings.HasSuffix(v, "rem") {
+			return fmt.Errorf("font size step %q must be a rem value, got %q", k, v)
+		}
+	}
+	return nil
+}
+
 // FontSizeOverrideKeys returns the canonical override keys, sorted.
 func FontSizeOverrideKeys() []string {
 	keys := make([]string, 0, len(fontSizeOverrideKeys))