feat: add public composable feedback flows

[neonwatty]

Summary

• adds the public window.BugDrop.registerFlow FlowConfig V1 API
• composes validated message, form, conditional, and screenshot screens
• submits through the established feedback payload while preserving the default BugDrop flow
• adds backwards-compatibility, accessibility, cancellation, concurrency, and browser coverage

Compatibility

The existing default widget and registerVariant behavior remain unchanged. The new flow runtime is additive and the default-shaped example is reconstructed with the new primitives.

Verification

• npm run validate: 88 files, 1,344 tests passed
• full Playwright suite: 317 passed across Chromium plus Radix compatibility coverage in Chromium, Firefox, and WebKit
• PR review toolkit run against 32ee17fbbf3f1dce617ac06042c25c5707dd8d94; accepted findings resolved and independently rechecked
• hardened Issue URL validation, Markdown code rendering, capture cancellation, attachment interleavings, and attachment validation

Related

The website lab PR pins the exact built widget from commit b307db46d895ae141f34befbc8cfcd107b155bc9: https://github.com/mean-weasel/bugdrop-web/pull/71