fix: retain X embed settings and prepare multimedia 2.1.0

[HMarzban]

X embeds lost their configured options when oEmbed failed. Keep those options in fallback markup and prepare @docs.plus/extension-hypermultimedia@2.1.0.

What

• Preserve X theme, width, language, privacy, alignment, and hidden-content settings in fallback markup.
• Add browser regressions for reported edit transactions, inline image links, text focus, and visible preview labels.
• Document image-link and WebView limits without claiming unverified Blazor or native-device support.
• Prepare the independent 2.1.0 package release, including the existing Vimeo hash fix and public isSafeMediaSrc export.
• Replace stale Context7 version claims with instructions to read package manifests and peer requirements.

The changelog covers every runtime source change since the 2.0.0 package tag, including the fullscreen-attribute fix and current dependency requirements.
The Vimeo contribution retains credit through legacy PR #4.

Validation

• Both new X fallback cases failed before the fix and pass afterward.
• Targeted browser checks pass for direct/chained hyperlink edits, narrow preview labels, inline image round-trip, text focus, audio/video serialization, and touch controls.
• Package and Cypress TypeScript checks pass.
• Normal pre-commit and pre-push hooks pass, including all five extension Cypress suites, unit tests, lint, styles, format, security, and both application builds.
• Package preflight and bun pm pack pass. The tarball resolves peer ranges and includes both module formats, declarations, CSS, changelog, README, and license.
• An independent Bun consumer passes ESM, CJS, strict TypeScript, CSS, and package-content checks.
• Production CI passed at 8a57773d0, including both application builds.
• Hosted chatroom tests passed: 31 tests, with one existing pending case. The image fixture now uses its real dimensions and waits for image readiness.

Backend service E2E and the webapp Cypress suite are outside this local quality gate and were not run for this change.
Publishing, source tagging, community replies, and closures remain separate maintainer actions. This PR does not activate family lockstep or create a GitHub Release.