fix(downloads): GIF review round 1 — shape, HDR, caps, audio, copy

- A GIF squares its pixels before the size, so an anamorphic source keeps
  its shown shape (a 720x480 DVD at 16:9 was 480x320).
- An editor GIF of an HDR upload is tonemapped, as a clip GIF is.
- The length caps are judged in whole milliseconds: a range exactly at a
  cap is no longer refused for a float hair over it.
- A GIF is made of a video only: an audio file is refused with its reason
  and the editor offers no GIF for it.
- A GIF clip keeps no audio track: the track neither splits its cache key
  nor fails it.
- The editor's GIF notes keep their live region mounted; one size reads
  as one ("about 1 MB"), none is shown with nothing kept; the size choices
  show their unit; a shared picture that fails to load says so.
- Copy: the two-sentence GIF notes keep their last stop (a copy-rule test
  now holds it); the English warns of "only a still picture".

frontend/src/watch/WatchPage.tsx

@@ -488,7 +488,14 @@ function SharedPicture({ meta, nested }: { meta: FileMeta; nested: boolean }) {
           width={meta.width ?? undefined}
           height={meta.height ?? undefined}
           retries={1}
-          fallback={<FileIcon className="m-12 w-12 h-12 text-muted" aria-hidden />}
+          // A picture link always has a picture: a missing one failed to load, and says so —
+          // the bare file glyph would read as a file with no preview.
+          fallback={
+            <div role="status" className="flex flex-col items-center gap-2 px-12 py-10 text-muted">
+              <FileIcon className="w-12 h-12" aria-hidden />
+              <p className="text-sm">{T.pictureFailed}</p>
+            </div>
+          }
           className={clsx(
             "max-w-full h-auto w-auto mx-auto block",
             nested ? "max-h-[60dvh]" : "max-h-[80dvh]",