feat: add mobile responsive flow tests

[somotochukwu-dev]

Summary

Add mobile-first responsive Playwright tests for the core learning and wallet flows. The tests run against both the chromium (desktop) and mobile-chrome (Pixel 7) Playwright projects, with mobile-specific assertions gated behind isMobile.

Changes

frontend/e2e/tests/responsive.spec.ts (new file)

13 tests across 5 concern areas, all using the existing web3.fixture for wallet mocking:

Home page — overflow check on any viewport, CTA reachability on mobile, feature card width containment on mobile.

Mobile navigation — hamburger menu toggle, link visibility on mobile, absence of hamburger on desktop (confirms xl:hidden breakpoint).

Wallet authentication prompt — wallet gate rendering without horizontal overflow on both viewports, button viewport containment on mobile (verifying touch targets are tappable).

Learning dashboard with wallet — dashboard overflow on both viewports, course selector / lesson controls reachability on mobile, Prev/Next footer controls on mobile.

Error and loading states — error boundary messages contain no stack traces, module paths, or file:line patterns on API failure, and the retry action ("Try again") is present. Loading skeleton renders without overflow.

Acceptance criteria coverage

• Selected flows pass at mobile viewports (navigation, wallet prompts, dashboard, courses)
• Controls remain reachable (bounding box checks confirm elements stay within viewport width)
• No horizontal overflow occurs (scrollWidth <= clientWidth checked on every page)
• Playwright tests cover navigation and wallet prompts (dedicated test suites for both)
• User-facing errors are actionable and do not expose internal details (error boundary assertions)
• The affected flow remains usable on supported desktop and mobile layouts (tests run against both Playwright projects with conditional isMobile skips)

Closes #932

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[ayomideadeniran]

pr under review