feat(mobile): set data-form-factor body attr from User-Agent in root layout

src/lib/form-factor.ts

@@ -0,0 +1,14 @@
+export type FormFactor = 'mobile' | 'desktop';
+
+const MOBILE_TOKENS = ['Mobile', 'iPhone', 'iPad', 'Android'] as const;
+
+/**
+ * Classify a User-Agent string as 'mobile' or 'desktop'.
+ * Defaults to 'desktop' when the UA is missing or unrecognized — the CSS
+ * media-query fallback in globals.css handles desktop browsers resized below
+ * the lg breakpoint, so a wrong-but-defaultish classification never breaks UX.
+ */
+export function classifyFormFactor(userAgent: string | null | undefined): FormFactor {
+  if (!userAgent) return 'desktop';
+  return MOBILE_TOKENS.some((token) => userAgent.includes(token)) ? 'mobile' : 'desktop';
+}