Test my signup flow at phone, tablet, and desktop sizes

Test the signup flow on [app URL] — or [optional: a different flow] — across three viewport sizes: phone (390px wide), tablet (768px), and desktop (1440px). At each size, go through the entire flow: the landing page CTA, the form itself, validation errors, and the post-signup screen. Screenshot every step. Look for layout breaks: clipped buttons, overlapping text, inputs that fall off-screen, modals that cannot be dismissed, and tap targets too small to hit. Also note anything that technically works but feels broken, like the primary CTA sitting below the fold on mobile. Report by viewport: what passed, what broke, screenshots of each problem, and severity — blocker (cannot complete signup), major (completable but painful), or cosmetic. End with the shortest list of fixes that would make all three sizes shippable.