fix: show a placeholder when an image cannot be loaded (#27730)

A message referencing a file that no longer exists rendered a broken
image, because nothing anywhere noticed the failed load. What the user saw
depended on the caller's alt text: in a response the alt is the message
content, so the entire reply was rendered inside the image frame, and the
preview still opened full-screen on a dead image.

Image.svelte now tracks a failed load and renders an 'Image unavailable'
placeholder instead, suppressing the preview for a source that cannot be
shown. The failed state resets when the source changes, so a replaced or
corrected URL is retried. An onError callback lets a caller react without
changing behaviour for the eight existing call sites, which are untouched.

Responses now pass the file name (or 'Generated Image') as alt rather than
the message content, which was never a description of the image.

src/lib/components/chat/Messages/ResponseMessage.svelte

@@ -690,7 +690,7 @@
 								{#each message.files.filter((f) => ['image', 'file'].includes(f.type)) as file}
 									<div>
 										{#if file.type === 'image' || (file?.content_type ?? '').startsWith('image/')}
-											<Image src={file.url} alt={message.content} />
+											<Image src={file.url} alt={file.name || $i18n.t('Generated Image')} />
 										{:else}
 											<FileItem
 												item={file}