feat(files): the Downloads page goes — the tray, a Storage tab and a Downloads settings tab take its place

- The Downloads module, its rail icon and badge are gone. A bookmark, the remembered page and an old
  history entry that name it land on Files (urlState.resolvePage); `g d` goes to Files too.
- Users page: a Storage tab — the disk in numbers, every user's use against their own limit with
  their file count, every user's downloads; aligned tables, one menu per row (edit / reset quota,
  open source, copy, the owner's quota). The admin storage rows carry quota + file count, the admin
  job list its owner.
- Settings: a Downloads tab — the download formats in one list (duplicate a built-in, edit or delete
  your own) and the site sign-ins, moved from Account.
- "View" on an added download, a clip or a converting upload opens the tray on that row (unfolded,
  back if closed; a finished one opens as a file; under a player it opens without taking focus).
- The download dialog opens on the format used last.
- DataTable: `pager={false}` for a short list, `cardRow` to give a lone "…" its own card line.
- Welcome and README speak of Files; a new Files screenshot.

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

@@ -70,13 +70,13 @@ afterEach(() => {
 describe("BootGate", () => {
   it("a new tab with Startup off opens on the page the account left — never on Home first", async () => {
     localStorage.setItem("siftlode.startOnHome.5", "0");
-    localStorage.setItem("siftlode.page.5", "downloads");
+    localStorage.setItem("siftlode.page.5", "plex");
     vi.spyOn(api, "me").mockResolvedValue(account());
     mount();
 
-    expect((await screen.findByTestId("app")).textContent).toBe("downloads");
-    expect(renders.map((r) => r.page)).toEqual(renders.map(() => "downloads"));
-    expect(new URLSearchParams(window.location.search).get("page")).toBe("downloads");
+    expect((await screen.findByTestId("app")).textContent).toBe("plex");
+    expect(renders.map((r) => r.page)).toEqual(renders.map(() => "plex"));
+    expect(new URLSearchParams(window.location.search).get("page")).toBe("plex");
     expect(sessionStorage.getItem(ACTIVE_ACCOUNT_KEY)).toBe("5"); // the tab is pinned
   });
 
@@ -176,12 +176,12 @@ describe("BootGate", () => {
     // Pinned to 9, but 9 was signed out in another tab: the server answers as the browser's default, 5.
     sessionStorage.setItem(ACTIVE_ACCOUNT_KEY, "9");
     localStorage.setItem("siftlode.startOnHome.5", "0");
-    localStorage.setItem("siftlode.page.5", "downloads");
+    localStorage.setItem("siftlode.page.5", "plex");
     vi.spyOn(api, "me").mockResolvedValue(account());
     mount();
 
-    expect((await screen.findByTestId("app")).textContent).toBe("downloads");
-    expect(renders.map((r) => r.page)).toEqual(renders.map(() => "downloads"));
+    expect((await screen.findByTestId("app")).textContent).toBe("plex");
+    expect(renders.map((r) => r.page)).toEqual(renders.map(() => "plex"));
     expect(sessionStorage.getItem(ACTIVE_ACCOUNT_KEY)).toBe("5");
   });