feat(desktop): say "attachment expired" instead of showing a broken file

[mattbalza]

What

A message can outlive its own attachment. Until now that read as a bug in the app rather than an old message:

• Images rendered a broken frame, still wrapped in a lightbox trigger that opened onto nothing.
• Files kept a download button that failed every single time it was pressed, with a toast that was gone before the next press.

Both now render an "Attachment expired" card that keeps the filename — so the message still records what was shared — and offers no action, because there is nothing left to fetch.

How each path learns it

They fail differently, so they ask differently:

surface	how it fails	how it asks
ImageBlock	<img> error event, which carries no status	one-byte range GET at the media URL, classify the response
FileCard	native download_file rejection	parse the status out of the message

The file path deliberately does not probe. href there is the unrewritten relay URL — RELAY_MEDIA_RE only matches image/video extensions, so a PDF is never proxied and a webview fetch to it is bounced by Cloudflare. That 403 would classify as unavailable and the card would never learn the truth. The rejection string already carries it: relay_error_message builds every relay error as relay returned {status}[: message], and that prefix is the stable half.

Only 404 and 410 are expiry

Everything else is a fault that may clear. Telling someone their file is gone because the network blinked is the one wrong answer available here, so 403, 429, 5xx and a thrown fetch all stay a plain failure with the normal toast. A message with no status in it at all — a refused connection, a cancelled save dialog, a hash mismatch — returns null and changes nothing.

Incidentally fixed

FileCard was doing err instanceof Error ? err.message : "Download failed". Tauri rejects with the command's raw String, never an Error, so every real download error was being discarded and replaced with the generic text. errorText() now handles both shapes.

Test plan

• desktop/src/shared/lib/mediaAvailability.test.mjs — 7 tests: 404/410 → expired, 200/206/304 → available, 403/429/5xx → unavailable, status parsed out of the relay wording, statusless messages → null, the probe's shape (GET + range: bytes=0-0 + no-store), and a throwing fetch → unavailable.
• npx tsc --noEmit -p tsconfig.json clean.
• npx biome check clean.