Poster/Talk preview popover with download actions (#840)

[jonfroehlich]

Closes #840.

What

Hovering or clicking a publication's Poster / Talk link now opens a Cite-style card showing the artifact thumbnail plus download actions — PDF, the raw file (PPTX/Keynote/…), and the editable Source link (external_slides_url) — each with its file size. Clicking the thumbnail opens the PDF.

Interaction & accessibility

• Opens on hover (mouse), and on click / Enter / tap (which pins it open). Mirrors the existing Cite popover.
• On click/keyboard open, focus moves into the card and Tab is trapped within it; Escape and outside-click close it and restore focus to the trigger.
• Touch: a tap opens the card (no hover), so touch users reach every action.
• Trigger keeps href to the PDF, so it still works with JS disabled (progressive enhancement).
• Degrades to the plain direct-to-PDF link when the artifact has no thumbnail.

Robustness fix

The card shows each file's size, read from storage at render time via FileField.size — which raises FileNotFoundError when a file is missing on disk. Because the size renders for every entry on a listing, a single missing file 500'd the entire /publications/ page. New safe pdf_file_size / raw_file_size properties on Artifact make a missing file degrade to "no size shown" instead. (Surfacing/fixing the underlying missing files is the existing Media / file integrity Data Health check's job.)

Files

• snippets/artifact_preview_link.html (new) — reusable trigger + inert <template> card; the image lazy-loads only when the card opens.
• static/website/js/thumbnailPreview.js (new) — self-contained, event-delegated popover (works with the member page's AJAX "load more" with no re-init).
• static/website/css/publications.css — card/action styling; reuses Bootstrap's .popover.
• models/artifact.py — safe file-size properties.
• display_pub_snippet.html + the 5 pages that render it.

Tests

website/tests/test_thumbnail_preview.py (8 tests): card markup, raw-file & Source actions, plain-link fallback, both layouts, and the missing-file degrade-not-crash path. Full suite green (the one unrelated test_page_metadata red is a stale assertion from 7e497aa, fixed separately).

Before merging

• Before/after screenshots (per CONTRIBUTING — UI change)
• Pa11y run on a publications page

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