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/DownloadCenter.confirm.dom.test.tsx

@@ -10,7 +10,9 @@ import type { DownloadJob } from "../lib/api";
 
 const mocks = vi.hoisted(() => ({
   jobs: [] as unknown[],
-  confirm: vi.fn(async (_opts: { title: string; confirmLabel?: string }) => false),
+  confirm: vi.fn(
+    async (_opts: { title: string; message?: string; confirmLabel?: string }) => false,
+  ),
 }));
 
 vi.mock("react-i18next", async (importOriginal) => {
@@ -141,4 +143,41 @@ describe("DownloadCenter confirm dialogs", () => {
       "downloads.confirm.cancelUploadAction",
     ]);
   });
+
+  it("cancelling a clip speaks of the clip", async () => {
+    mocks.jobs = [{ ...job(4, "running"), job_kind: "edit" }];
+    render(
+      <QueryClientProvider client={new QueryClient()}>
+        <DownloadCenter />
+      </QueryClientProvider>,
+    );
+    await pick("Clip 4", "downloads.actions.cancel");
+    await waitFor(() => expect(mocks.confirm).toHaveBeenCalledTimes(1));
+    const [opts] = mocks.confirm.mock.calls[0]!;
+    expect([opts.title, opts.message, opts.confirmLabel]).toEqual([
+      "downloads.confirm.cancelEditTitle",
+      "downloads.confirm.cancelEditBody",
+      "downloads.confirm.cancelEditAction",
+    ]);
+  });
+
+  it.each([
+    ["upload", "downloads.confirm.deleteUploadTitle", "downloads.confirm.deleteUploadBody"],
+    ["edit", "downloads.confirm.deleteEditTitle", "downloads.confirm.deleteEditBody"],
+    ["download", "downloads.confirm.deleteTitle", "downloads.confirm.deleteBody"],
+  ] as const)("deleting a finished %s says what happens to its file", async (kind, title, body) => {
+    // Only a download's file may stay cached for other users; an upload's or a clip's is theirs alone.
+    mocks.jobs = [{ ...job(5, "done"), job_kind: kind }];
+    render(
+      <QueryClientProvider client={new QueryClient()}>
+        <DownloadCenter />
+      </QueryClientProvider>,
+    );
+    fireEvent.click(screen.getByRole("tab", { name: /downloads\.tabs\.done/ }));
+    await screen.findByText("Clip 5");
+    await pick("Clip 5", "downloads.actions.delete");
+    await waitFor(() => expect(mocks.confirm).toHaveBeenCalledTimes(1));
+    const [opts] = mocks.confirm.mock.calls[0]!;
+    expect([opts.title, opts.message]).toEqual([title, body]);
+  });
 });