a11y | A16 image alt from prompt and attachment labels

[BrianGenisio]

Summary

Closes #70 (A16). Chat images used filenames or empty markdown alt, so a generated picture often had no useful description for assistive tech.

Product rule: assistant/generated images use the previous user message. That string is available both live and after reload (tool-call input is not persisted). User uploads and composer thumbnails are labelled as attachments.

Changes

• User file parts and composer thumbs: t('Attached image: {name}'), with t('attachment') when the filename is missing.
• Assistant file parts and empty markdown ![](): previous user message text, otherwise t('Generated image').
• Non-empty markdown alt is kept.
• Alt is escaped in HTML strings, or assigned via the img property on composer thumbs.

Look at renderFilePart, renderer.image, and how previousUserText is threaded through renderMessages. The tool-call prompt is intentionally unused; this repo never reads tool-call input, and serializeLiveMessages drops those parts on save.

Test plan

• npm test
• Upload an image in the composer and in a sent user bubble: alt should be Attached image: {filename}
• Generate an image: VoiceOver should read the prompt that requested it, not the filename
• Reload the session: same alt still present (previous user message remains; tool-call does not)
• Older markdown ![]() replies: empty alt falls back to the previous user message; explicit ![description]() is kept
• Spanish catalog: Imagen adjunta: {name} / Imagen generada
• Confirm axe baseline does not grow

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📥 Commits

Reviewing files that changed from the base of the PR and between ef43728 and 4cf8444.

📒 Files selected for processing (5)

• a11y-audits/8-5-26/wave-4-plan.md
• i18n/en.json
• i18n/es.json
• public/app.js
• tests/dom/render.test.js

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