feat(web): accessible UI rewrite (v1.8.0)

[CryptoJones]

Summary

• rebuild the build-free SPA around semantic landmarks, native controls, focus management, live announcements, and accessible dialogs
• replace legacy themes with system/light/dark, high-contrast, reduced-motion, text-size, and density preferences
• add Playwright + axe WCAG checks, CI enforcement, accessibility documentation, and an issue template
• update desktop/mobile screenshots and bump both runtimes to 1.8.0

Compatibility

• preserves every existing API endpoint and JSON shape
• adds no database migrations
• keeps the production SPA unbundled and self-hostable

Verification

• npm run test:web (10 passed, 2 intentional project-specific skips)
• npm audit --audit-level=high (0 vulnerabilities)
• pytest -q (55 passed)
• ruff check src tests
• mypy src
• dotnet build --no-restore
• 60 non-containerized .NET tests passed locally
• full Testcontainers-backed .NET suite runs in GitHub CI