Use @gryt/core instead of this app's copy

[sivert-io]

612 lines out, and the first half of making the two apps 1:1. The mobile side is
the same change against the same package.

What moved

The report payload, the link preview provider table, the card layout rule and
the failure wording. Every one had a twin in the phone app, and
@gryt/core@0.1.0 is now the single
implementation.

What stayed, and why

Both for the same reason — they need a platform, and the package compiles
without one.

embedProviderIcons.ts keeps the artwork. A logo is a React component here
and an SVG path or a favicon on the phone. The package owns hostnames, colours
and path rules; each app maps a provider id to its own icon.

embedUtils.ts keeps what's only true of a desktop: which sites get a real
player rather than a card, and the dismissal and cache state a session holds. It
re-exports the moved names, so this cost one import line rather than a rename
across six files.

lib/reports/diagnostics.ts stays. It's what this app can find out about
itself — Electron version, Chrome build, the embedded server — and the package
holds the shape those fill in.

Worth a look

check-embed-icons.mjs replaces check-embed-cards.mjs. The old one tested
layout rules and hostname matching, which are now tested in the package, once
instead of twice. The new one watches the seam the package can't: it fails when
@gryt/core knows a site this app can't draw, and when this app draws one the
package has dropped. Currently 73 providers, all drawn.

One behaviour change, and it lands on the phone rather than here.
MESSAGE_MAX was already 8000 in this app. Mobile had 4000, and the package
took 8000 — so bug report text gets more generous on the phone, not less
anywhere.

The re-export needed an import next to it. export { type LinkPreviewData } from "@gryt/core" doesn't put the name in the module's scope, and previewCache
below it is typed with it. That's the one thing typecheck caught, and it would
have been a runtime-clean silent any if the file weren't strict.

Checks

tsc -b clean for every changed file, lint clean, yarn test:embed-icons
passes. I verified all of this against a packed tarball of the exact artifact
before it was published, then again against the real thing from the registry.