Preserve images when pasting HTML content

[T9knauf]

Closes #922

IssueHunt bounty: https://issuehunt.io/repos/74213528/issues/922

This adds focused HTML clipboard conversion when pasted content includes images. Existing file paste/upload handling still runs first, and HTML fragments without images continue through CodeMirror's normal paste behavior.

For pasted web content containing <img> tags, the handler converts the clipboard HTML to Markdown and preserves image URLs as Markdown image links instead of trying to download remote assets, matching the maintainer guidance in #922.

Test plan:

• ./node_modules/.bin/jest -c jest.json --runInBand src/cloud/lib/editor/htmlToMarkdown.spec.ts
• ./node_modules/.bin/eslint src/cloud/lib/editor/htmlToMarkdown.ts src/cloud/lib/editor/htmlToMarkdown.spec.ts src/cloud/lib/editor/plugins/fileHandler.ts --ext .ts,.tsx
• ./node_modules/.bin/tsc --noEmit --pretty false
• git diff --check

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