fix(files): review round 1, the page — honest states, the keyboard, the tray's words

- A library past the list's cap is searched and sorted on the server; the count says "the newest".
- Each folder and each search opens on its first page (DataTable pageScope, useCardPager scope).
- A file dropped where the page takes none never reaches the browser (which would unload the app).
- The viewer's arrows wait while a layer opened from it is on top, and scroll a wide text line;
  a text shows its first 5000 lines and says where the rest is.
- Confirms name the file (no more "your downloads"); the asking menu rows end in "…".
- The tray names a running step beside its bar, shows a pause mark for paused work, polls slowly
  while only paused work waits, and forgets a finished file that was deleted.
- An unchanged rename sends nothing; a paste opens the download dialog only with nothing on top;
  the link box never replaces what was typed.
- Honest states: folders that failed to load, a file no longer there, the storage figure; the rail
  says "nothing downloaded" only when there is nothing.
- Focus after deleting the open folder and when the format form opens; the format name's label.
- ui/Meter (one danger line for both quota bars) and ui/RenameField in the platform layer.
- Copy fixes, stale comments and the speculative hub key; a contract test for the list's
  parameters; e2e: the Files page and the paste intake.

frontend/src/components/FilesRail.dom.test.tsx

@@ -155,3 +155,31 @@ describe("FilesRail · folders", () => {
     await waitFor(() => expect(document.activeElement).toBe(entry("Receptek")));
   });
 });
+
+describe("FilesRail · what it says when a part is empty or missing", () => {
+  it("says nothing was downloaded only when nothing is there — not when it all moved into own folders", async () => {
+    await mount([folder(1, { source_key: "youtube", count: 0 }), { ...OWN, count: 3 }]);
+    expect(screen.queryByText("files.rail.noSources")).toBeNull();
+    cleanup();
+    await mount([folder(1, { source_key: "youtube", count: 0 }), { ...OWN, count: 0 }]);
+    expect(screen.getByText("files.rail.noSources")).toBeTruthy();
+  });
+
+  it("a storage figure that failed to load says so, rather than the meter quietly not being there", async () => {
+    vi.spyOn(api, "fileFolders").mockResolvedValue([YT, OWN]);
+    vi.spyOn(api, "filesUsage").mockRejectedValue(new Error("down"));
+    render(
+      <QueryClientProvider
+        client={new QueryClient({ defaultOptions: { queries: { retry: false } } })}
+      >
+        <FilesRail
+          layout={defaultLayout("files")}
+          setLayout={vi.fn()}
+          collapsed={false}
+          onToggleCollapse={vi.fn()}
+        />
+      </QueryClientProvider>,
+    );
+    expect(await screen.findByText("files.usage.failed")).toBeTruthy();
+  });
+});