feat(files): folders managed in place, a grid per folder, drop to upload, open on one file

- Own folders are made, renamed and deleted from the rail (right click, long press,
  Shift+F10) or the "…" beside an open folder's name; any folder's retention is a
  submenu (7/30/90 days, until deleted). Deleting confirms where its files go.
- List or grid per folder, sharing one sort; the grid is the one media card.
- A file dropped anywhere on the page opens Upload with it, bound for the folder
  on screen.
- A link (?item=) or the upload notice opens Files on the file: its page turned
  to, the keyboard on it, a ring round it.
- An in-place name field left by Enter or Escape hands the focus back to what
  replaced it (the row, the entry, the "…") instead of dropping it on <body>.
- The table's saved page size is per account.

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

@@ -0,0 +1,92 @@
+// @vitest-environment jsdom
+//
+// FilesProvider's arrival at one file: `openFile` (a notice's "Open") and a `?item=` link both make a
+// pending focus that the Files page spends once it has looked; what it found is held until the page
+// is left. Real storage with the tab's account pinned; only the navigation is mocked, its page
+// switchable.
+import { beforeEach, describe, expect, it, vi } from "vitest";
+import { act, renderHook } from "@testing-library/react";
+import type { ReactNode } from "react";
+
+const nav = vi.hoisted(() => ({ page: "files", setPage: vi.fn() }));
+vi.mock("./navigationContext", () => ({
+  useNavigation: () => ({ page: nav.page }),
+  useNavigationActions: () => ({ setPage: nav.setPage }),
+}));
+
+import { FilesProvider } from "./FilesProvider";
+import { ALL_FILES, useFiles, useFilesActions } from "./filesContext";
+
+const wrapper = ({ children }: { children: ReactNode }) => (
+  <FilesProvider>{children}</FilesProvider>
+);
+const render = () => renderHook(() => ({ ...useFiles(), ...useFilesActions() }), { wrapper });
+
+beforeEach(() => {
+  localStorage.clear();
+  sessionStorage.setItem("siftlode.activeAccount", "1");
+  window.history.replaceState(null, "", "/");
+  nav.page = "files";
+  nav.setPage.mockClear();
+});
+
+describe("FilesProvider · arriving at one file", () => {
+  it("openFile opens its folder on the Files page, with no search hiding it", () => {
+    nav.page = "feed";
+    const { result, rerender } = render();
+    act(() => result.current.setSearch("valami"));
+    act(() => result.current.openFile(7, 3));
+    expect(nav.setPage).toHaveBeenCalledWith("files");
+    expect(result.current.view).toEqual({ kind: "folder", id: 3 });
+    expect(result.current.search).toBe("");
+    nav.page = "files";
+    rerender();
+    expect(result.current.focus).toMatchObject({ id: 7 });
+    // The address carries it until it is spent, so a reload mid-arrival still lands on the file.
+    expect(new URLSearchParams(window.location.search).get("item")).toBe("7");
+  });
+
+  it("a file in no folder opens every file", () => {
+    const { result } = render();
+    act(() => result.current.openFile(7, null));
+    expect(result.current.view).toEqual(ALL_FILES);
+  });
+
+  it("a ?item= link is an arrival from the start", () => {
+    window.history.replaceState(null, "", "/?item=12");
+    const { result } = render();
+    expect(result.current.focus).toMatchObject({ id: 12 });
+  });
+
+  it("spent, it is what the page found — until the page is left", () => {
+    const { result, rerender } = render();
+    act(() => result.current.openFile(7, null));
+    act(() => result.current.consumeFocus("all"));
+    expect(result.current.focus).toBeNull();
+    expect(result.current.arrived).toMatchObject({ id: 7, view: "all" });
+    expect(new URLSearchParams(window.location.search).get("item")).toBeNull();
+    nav.page = "feed";
+    rerender();
+    expect(result.current.arrived).toBeNull();
+    nav.page = "files";
+    rerender();
+    // Back on the page later: nothing is rung or turned to again.
+    expect(result.current.arrived).toBeNull();
+  });
+
+  it("an arrival found nowhere leaves nothing found", () => {
+    const { result } = render();
+    act(() => result.current.openFile(7, null));
+    act(() => result.current.consumeFocus(null));
+    expect(result.current.focus).toBeNull();
+    expect(result.current.arrived).toBeNull();
+  });
+
+  it("the same file opened again is a new arrival", () => {
+    const { result } = render();
+    act(() => result.current.openFile(7, null));
+    act(() => result.current.consumeFocus("all"));
+    act(() => result.current.openFile(7, null));
+    expect(result.current.focus).toMatchObject({ id: 7 });
+  });
+});