UI: add parity, accessibility, performance, and release conformance

Parent: #5451
Related API conformance: #5485

Summary

Establish and execute the final parity, browser E2E, accessibility, security, performance, route-prefix, and release matrix for the required Mantine UI surface.

Service-specific tests remain in their feature issues. This issue proves that the complete required UI works as a bundled Alertmanager application.

Parity matrix

Maintain a checklist against Elm for:

• Alerts, natural/custom grouping, receiver selection, state and label filters, annotations, labels, links, times, expansion, and Silence actions.
• Silence active/pending/expired lists, counts, filters, detail, create, update, annotations, preview, expire/delete, and resulting IDs.
• Status, Configuration, Settings, local preferences, navigation, errors, loading, empty states, and deep links.
• Root/non-root route prefixes and links between Classic and New UI.

Every missing item must be implemented or recorded as an explicit maintainer-approved exception. Optional NotificationService UI is excluded.

Browser and accessibility coverage

• Add Playwright tests against a real built Alertmanager binary.
• Cover /ui/ and a non-root route prefix, direct nested loads, refreshes, route transitions, Connect unary calls/mutations, feature-disabled/enabled errors, reloads, and watch reconnect/resnapshot.
• Add automated accessibility checks plus explicit keyboard navigation, focus order/visibility, announcements, zoom, and mobile viewport scenarios.
• Exercise auto/light/dark themes and persisted settings.

Performance and security

• Generate large alert/group/silence datasets and measure initial render, expansion, filtering, pagination, and browser memory.
• Avoid rendering every collapsed alert; add windowing/virtualization where measurement requires it.
• Test labels, annotations, comments, configuration, matchers, and URLs containing hostile, escaped, and UTF-8 values without unsafe HTML or broken links.
• Verify generator/external links use safe target behavior and relative URLs resolve intentionally.
• Verify runtime bootstrap remains CSP-compatible and same-origin authentication works.

Build and release conformance

• Verify clean nested routes and assets under root and non-root route prefixes.
• Verify dynamic HTML caching, immutable asset caching, identity/gzip/Brotli responses, missing-asset 404s, and third-party license attribution.
• Verify common CI and release architectures embed both UIs.
• Verify both prebuilt asset artifacts and their documented extraction workflows.
• Document /ui/, continued Classic UI availability, v3alpha/feature limitations, and troubleshooting.
• Add staged changelog entries for user-visible delivery.

Acceptance criteria

• Every required parity item passes or has a documented maintainer-approved exception.
• Playwright passes against a real binary at /ui/ and a non-root route prefix, including nested refreshes.
• Automated accessibility and explicit mobile/keyboard/focus checks pass.
• Large datasets meet agreed rendering and memory thresholds.
• Hostile/escaped/UTF-8 content remains safe and functional.
• CSP-compatible bootstrap, same-origin auth, caching, compression, licenses, and release assets are verified.
• Generated output is reproducible and a second generation produces no diff.
• Mantine typecheck/check/tests/build, Go UI tests, make common-format, make lint, make test-short, and make test pass.
• Optional notification observability does not block completion.

Dependencies and related work

• Required UI children: #5486, #5487, #5488, #5489, #5490, #5491, and #5492.
• Coordinate browser/API conformance with #5485.
• Large-alert considerations relate to #924.
• Relevant security/path backlog includes #4509, #4499, #2340, #1881, and #1826.

Likely code areas

• Playwright configuration and tests under ui/mantine-ui/
• component accessibility/performance fixtures
• ui/web_test.go, test/e2e/**, and CI workflows
• README.md, CHANGELOG.md, and UI contributor documentation