demo-ui: add Playwright end-to-end, accessibility, and responsive tests

[teesofttech]

Why

The demo UI is itself evidence that providers work. It needs repeatable browser tests, accessible interaction, and deterministic mock-provider paths in CI.

Scope

• Add mock provider adapters for success, pending, decline, timeout, and refund.
• Test provider selection, validation, redirect return, verification, webhook timeline, and refunds.
• Run automated accessibility checks and keyboard-only flows.
• Cover mobile and desktop viewports.
• Capture screenshots/traces only on failure and sanitize them.

Acceptance criteria

• CI runs deterministic browser tests without live credentials.
• No serious WCAG 2.1 AA automated violations remain.
• Critical flows work by keyboard and screen reader labels are meaningful.
• Mobile layout is covered.
• Failure artifacts contain no secrets or PII.