feat: Error UX

Replace “preview unavailable” with actionable causes (HEIF, auth, too large, timed out) on the image popup modal

Search the repo for the literal text “preview unavailable” and identify the image popup modal component that displays it. Read that component and any related localization/string files to replace the message with the listed actionable causes (HEIF, auth, too large, timed out). Then run the project’s frontend test/build path and manually exercise the image preview failure flow to confirm the modal now shows the new wording.

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