Fix: show a fallback for slide thumbnails that fail to load

[posthog-eu]

Problem

• Who is hurt: a user preparing a live presentation sees the browser's broken-image icon in the slide grid instead of a slide preview, and cannot recover in the app.
• The grid draws each thumbnail as a bare <img> (FillLayer) with no onError, retry, or placeholder.
• Imported slides publish their media names before the medias__pdf_to_images worker finishes uploading the pages, so /media/processed answers 404 for a few seconds. The failure is silent: a failed <img> fires no event.

Changes

• FillLayer now retries a failed image fetch a few times with a growing delay (the media may still be uploading), then falls back to a placeholder icon instead of the broken-image glyph. Successful loads are unchanged.
• Slide grid skeletons any slide whose thumbnail name is not published yet, matching the guard the renderer path (ImageRenderer) already applies.

State	Before	After
Media still uploading (404)	Broken-image icon	Retry, then placeholder icon
Thumbnail name not published	Broken render / empty name	Skeleton
Media loads normally	Image	Image (unchanged)

Testing

• Added a FillLayer component test: repeated load failures drop the bare <img> and render the fallback icon; a normal load keeps the image.
• @repo/layout build (type-checked) and plugins/slides type-check pass.

Agent context

• I could not launch the full Electron app here (needs Postgres + backend), so the visual state change is confirmed through the real-DOM component test rather than a live screenshot.
• The scope matches the report: two files plus a test, reusing the renderer's existing empty-thumbnail guard. I did not touch the import worker's Promise.all upload path.

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