Implement Release 2 sharing and API tools

[alianjum0]

Summary

• add Slack and WhatsApp live previews
• expose a versioned JSON analysis endpoint with safe errors
• add reproducible audit links plus JSON and Markdown exports
• document the public API and expand syntax verification

Validation

• pnpm run verify — 51 tests passed, syntax checks passed, no known production dependency vulnerabilities
• desktop and 390x844 browser verification passed
• live Slack/WhatsApp updates, JSON smoke, Markdown download, and clean browser console verified
• staged diff whitespace check passed

Risks and limits

• exports rerun the target URL and may change with the source page
• share/export controls are URL-only because pasted HTML is not persisted
• this PR does not deploy the application or implement Release 3