Give readable content its own text size (#3637)

* feat(app): separate content typography from interface size

Readable prose benefits from its own scale without inflating navigation and controls. Preserve existing users’ rendered size during migration while giving fresh desktop installs a 15px content default.

* test(app): cover independent content typography

The appearance journey now verifies that changing interface size leaves content size intact. Timeline traversal yields when a test deliberately holds the requested page, so the helper no longer waits on its own gate.

* test(desktop): cover independent content typography

The desktop appearance regression follows the renamed Interface control and verifies it does not alter Content size.

packages/app/src/components/appearance-style-boundary.tsx

@@ -11,8 +11,8 @@ function AppearanceStyleBoundaryBase({ appearanceKey, children }: AppearanceStyl
   return <Fragment key={appearanceKey}>{children}</Fragment>;
 }
 
-const appearanceStyleBoundaryMapping = (theme: Theme): Partial<AppearanceStyleBoundaryProps> => ({
-  appearanceKey: [
+export function appearanceStyleBoundaryKey(theme: Theme): string {
+  return [
     theme.fontFamily.ui,
     theme.fontFamily.mono,
     theme.fontSize.sm,
@@ -22,6 +22,7 @@ const appearanceStyleBoundaryMapping = (theme: Theme): Partial<AppearanceStyleBo
     theme.fontSize["2xl"],
     theme.fontSize["3xl"],
     theme.fontSize["4xl"],
+    theme.fontSize.content,
     theme.fontSize.code,
     theme.lineHeight.diff,
     theme.colors.foreground,
@@ -51,8 +52,12 @@ const appearanceStyleBoundaryMapping = (theme: Theme): Partial<AppearanceStyleBo
     theme.colors.syntax.meta,
     theme.colors.syntax.heading,
     theme.colors.syntax.link,
-  ].join("\u0000"),
-});
+  ].join("\u0000");
+}
+
+function appearanceStyleBoundaryMapping(theme: Theme): Partial<AppearanceStyleBoundaryProps> {
+  return { appearanceKey: appearanceStyleBoundaryKey(theme) };
+}
 
 const ThemedAppearanceStyleBoundary = withUnistyles(AppearanceStyleBoundaryBase);