fix(view,text-editor): give the blob read path a failure state

[clayrisser]

Problem

Three places on the blob read path have no failure state at all. In each case a failed fetch leaves the UI stuck rather than showing the "Failed to preview" state the codebase already has.

1. TextViewer.svelte:24-36 awaits fetch with no try/catch:

async function fetchFile (value: Ref<Blob>, name: string): Promise<void> {
  loading = true

  const src = getFileUrl(value, name)
  const res = await fetch(src)
  text = await res.text()

  loading = false
}

If the fetch rejects, loading = false is never reached — so the spinner runs forever — and the rejection is unhandled. A non-2xx response is worse than that: it does not reject, so res.text() succeeds and the error body is rendered as the file's contents.

2. ImageViewer.svelte has on:load but no on:error. Same eternal spinner: _setLoading(false) only ever runs from on:load, so a broken image leaves the loading state set and the <img> pinned at height: 0.

3. embed.ts:122 has .then and .finally but no .catch:

void providerPromise
  .then((view) => {
    view = view ?? StubEmbedNodeView

A rejected provider promise is an unhandled rejection, and the nodeview never falls back to StubEmbedNodeView — so the embed renders as nothing at all, rather than as the stub link it is designed to degrade to.

Fix

Each path now settles into a state that already exists in the codebase — presentation.string.FailedToPreview for the two viewers (already defined at packages/presentation/src/plugin.ts:138 and translated in lang/en.json, so no new i18n), and the existing stub view for the embed.

• TextViewer — wrap in try/catch/finally, treat !res.ok as an error rather than rendering the body, and add an {:else if error} branch showing FailedToPreview.
• ImageViewer — add error state, an on:error handler on the <img>, and an error branch. The error flag resets alongside _setLoading(true) when value changes, so navigating from a broken image to a good one recovers.
• embed.ts — add a .catch that warns and returns undefined, which the existing view = view ?? StubEmbedNodeView line then turns into the stub.

Scope and residual risk

• No behaviour change on the success path in any of the three.
• The ImageViewer diff looks larger than it is: adding the {#if error} branch re-indents the existing DrawingBoard block into the {:else}. The only substantive changes are the error variable, the on:error handler, and the reset.
• TextViewer now treats a non-2xx response as a failure rather than rendering the response body as file contents. That is a deliberate behaviour change and the one thing here worth a second opinion — it is the right call for a file viewer, but it does mean a server that returns a 404 page with useful text will now show "Failed to preview" instead of that text.
• The two console.warns are the only new output. If you would rather they went through a measure context, say so and I will change them.

Verification

Verified against develop @ 1be6047c8: TextViewer.svelte:33 is still a bare await fetch, ImageViewer.svelte still has zero on:error handlers, and embed.ts:122 still has no .catch on providerPromise.

No automated tests. plugins/view-resources and plugins/text-editor-resources have no jest projects, and all three behaviours are DOM-level failure paths in Svelte components. Manual check for each: point the viewer at a blob id that does not resolve and confirm the "Failed to preview" label appears instead of a permanent spinner; give the embed node a URL whose provider match rejects and confirm the stub link renders.

Note on scope

This was originally prepared together with an absolute-URL guard for getPreviewThumbnail on the same read path. I have split that into a separate PR — it is a different concern, and it ships with unit tests whereas this one cannot.

[clayrisser]

Closing — this was opened by an automated agent without my intent. Apologies for the noise.