feat(files): a file that is not media opens in a viewer — an image, a PDF, text as text

- GET /api/files/{id}/preview: an allow-list on the stored extension (downloads/preview.py) picks
  image / PDF / text; served inline under `CSP: sandbox; frame-ancestors 'self'`, the type from the
  table; HTML and SVG arrive as plain text; a text is its first 1 MiB (X-Preview-Truncated), sent
  as UTF-8 whatever it was written in (BOM, UTF-8, else cp1250); a NUL means binary: 415
- the list says which preview each file gets (asset.preview); FE/BE contract on kinds and header
- the viewer (shared dialog shell): ← / → through the list's such files in its order, Save, open in
  a tab, the file's menu; one stage height for every kind; a no-preview card offers the file
- the name, the card's picture and the menu's Preview open it; an image up to 8 MB is its own
  thumbnail

frontend/src/components/Files.menu.dom.test.tsx

@@ -402,6 +402,62 @@ describe("Files · a shared file's row", () => {
   });
 });
 
+describe("Files · the viewer", () => {
+  const doc = (id: number, preview: string | null = "pdf") =>
+    file(id, {
+      job_kind: "upload",
+      display_name: `Doc ${id}`,
+      asset: { media_kind: "file", preview, container: "pdf", size_bytes: 10 },
+    } as never);
+  const viewer = () => screen.findByRole("dialog");
+
+  it("opens a file that is not media from its name, in the sandboxed preview", async () => {
+    mocks.files = [file(1), doc(2)];
+    mocks.folders = [YT];
+    mount();
+    fireEvent.click(screen.getByText("Doc 2"));
+    const dialog = await viewer();
+    expect(dialog.querySelector("iframe")!.getAttribute("src")).toBe(api.previewUrl(2));
+  });
+
+  it("puts Preview first in such a file's menu, and it opens the viewer", async () => {
+    mocks.files = [doc(2, null)];
+    mocks.folders = [YT];
+    mount();
+    await openMenu(() => fireEvent.contextMenu(rowOf("Doc 2")));
+    expect(menuRows()[0]).toBe("files.actions.view");
+    fireEvent.click(menuItem("files.actions.view"));
+    expect(await viewer()).toBeTruthy();
+    expect(screen.getByText("files.preview.none")).toBeTruthy();
+  });
+
+  it("steps through the list's files that are not media, in the list's order", async () => {
+    mocks.files = [doc(2), file(1), doc(3)];
+    mocks.folders = [YT];
+    mount();
+    fireEvent.click(screen.getByText("Doc 2"));
+    const dialog = await viewer();
+    fireEvent.keyDown(window, { key: "ArrowRight" });
+    await waitFor(() =>
+      expect(dialog.querySelector("iframe")!.getAttribute("src")).toBe(api.previewUrl(3)),
+    );
+  });
+
+  it("leaves Preview and Rename out of the viewer's own menu", async () => {
+    mocks.files = [doc(2)];
+    mocks.folders = [YT, OWN];
+    mount();
+    fireEvent.click(screen.getByText("Doc 2"));
+    const dialog = await viewer();
+    await openMenu(() =>
+      fireEvent.click(dialog.querySelector<HTMLElement>('button[aria-label="files.rowMenu"]')!),
+    );
+    expect(menuRows()).not.toContain("files.actions.view");
+    expect(menuRows()).not.toContain("files.actions.rename");
+    expect(menuRows()).toContain("files.actions.move");
+  });
+});
+
 describe("Files · the view", () => {
   it("starts the list at the scroller's top, so its sticky header does not jump on the first scroll", () => {
     mocks.files = [file(2)];