fix(downloads): a conversion's commit, a stopped upload, the fast join and a clip's copy hold up

- The fast join gives each ffconcat entry its own media-only format whitelist, so a library file
  can no longer be opened as a nested concat list (the list's whitelist was handed down to it).
- A file the guarded probe cannot read is a download-only file, not a guess from yt-dlp's info.
- A conversion's or an ingest's commit that raises is re-read in a fresh session before any file
  is removed: a commit that landed keeps the file its row now names.
- An original that cannot be removed after conversion is logged; the upload stays ready.
- An upload whose conversion is cancelled or deleted midway is freed once nothing holds it; a
  paused one is kept.
- A shared link says "preparing" only while its video's job is queued or running.
- The admin storage view counts every file on disk ("Files on disk"), not only ready ones.
- A clip's failure title, unknown failure and cancel dialog, and a clip's and an upload's delete
  dialog speak of what the job made; an upload's quota refusal explains that the original is kept.
- /watch announces "preparing" as an alert and moves focus to the new Retry after an action.

frontend/src/components/WatchPage.tsx

@@ -114,6 +114,10 @@ export default function WatchPage() {
 
   // Bumped by Retry, which asks again.
   const [attempt, setAttempt] = useState(0);
+  // Set once the viewer acted (Retry, the password): the state that answers replaces the button they
+  // were on, so its own Retry takes the focus rather than leaving it on <body>. Not on the first load,
+  // where nothing had the focus to lose.
+  const [acted, setActed] = useState(false);
   useEffect(() => {
     if (!token) return; // the seed above already says "gone"
     send(`/api/public/watch/${encodeURIComponent(token)}`)
@@ -136,6 +140,7 @@ export default function WatchPage() {
       .catch(() => setStatus("error"));
   }, [token, attempt]);
   const retry = () => {
+    setActed(true);
     setStatus("loading");
     setAttempt((n) => n + 1);
   };
@@ -167,6 +172,7 @@ export default function WatchPage() {
   };
 
   const unlock = async () => {
+    setActed(true);
     setUnlocking(true);
     setError(null);
     try {
@@ -203,11 +209,19 @@ export default function WatchPage() {
 
         {status === "preparing" && (
           <div className="flex-1 grid place-items-center text-center">
-            {/* Announced like the failure below: it replaces the loading line. */}
-            <div role="status">
+            {/* An alert, like the failure below: it is inserted already filled, which a polite status
+                region is not reliably announced for — and a Retry that answers "still preparing"
+                would otherwise say nothing at all. */}
+            <div role="alert">
               <div className="text-lg font-semibold">{T.preparing}</div>
               <div className="text-sm text-muted mt-1">{T.preparingHint}</div>
-              <Button variant="primary" size="md" onClick={retry} className="mt-4">
+              <Button
+                variant="primary"
+                size="md"
+                onClick={retry}
+                autoFocus={acted}
+                className="mt-4"
+              >
                 {T.retry}
               </Button>
             </div>
@@ -220,7 +234,13 @@ export default function WatchPage() {
             <div role="alert">
               <div className="text-lg font-semibold">{T.failed}</div>
               <div className="text-sm text-muted mt-1">{T.failedHint}</div>
-              <Button variant="primary" size="md" onClick={retry} className="mt-4">
+              <Button
+                variant="primary"
+                size="md"
+                onClick={retry}
+                autoFocus={acted}
+                className="mt-4"
+              >
                 {T.retry}
               </Button>
             </div>