[MT-23016] Resolve inline CID images safely in HTML previews

[kyrylo-yevdokymov]

What & why

Fixes MT-23016, a sub-task of MT-22849.

Browsers cannot resolve MIME cid: references directly. The preview now maps known content IDs in HTML URL attributes and CSS url() values to the existing attachment-part endpoint. Matching is intentionally limited to URL contexts so visible message text is never rewritten.

Impact

Inline images render in the HTML preview and popout while HTML Source remains unchanged.

Screenshots

Before	After

Verification

• npm test — 67 tests passed
• npm run lint — 0 errors; 4 existing warnings
• npm run build — passed
• git diff --check main...fix/mt-23016-inline-cid-images — passed

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Configuration used: Organization UI

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📥 Commits

Reviewing files that changed from the base of the PR and between 4c8f262 and 55a3d14.

📒 Files selected for processing (4)

• frontend/src/lib/inlineImages.test.ts
• frontend/src/lib/inlineImages.ts
• frontend/src/pages/MessageView.test.tsx
• frontend/src/pages/MessageView.tsx

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[kyrylo-yevdokymov]

@coderabbitai review

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[roman-adamchik]

lgtm