feat(files): the Files page — the library in one aligned list, its folders in a rail

Files is the one library: every done file, the ones shared with me, or one
folder's, picked in a side rail (sources, own folders, the storage meter) and
mirrored to ?folder=. One table with every kind of value in its own column;
Copy link, Share and Save in sight, every action on a right click: play,
cut, rename in the row, move to an own folder or back home, keep, details,
re-download, delete.

The Downloads page keeps only the queue until the tray replaces it. The old
/api/downloads library routes are gone: list shared, upload, the details
PATCH, delete, the file and the usage now live under /api/files, and a
listed file says which source it came from. The job's "expired" flag is gone
with the state it described.

frontend/src/components/FilesProvider.tsx

@@ -0,0 +1,55 @@
+import { useCallback, useMemo, useState, type ReactNode } from "react";
+import { LS, getAccountRaw, setAccountRaw } from "../lib/storage";
+import { useUrlMirror } from "../lib/useUrlMirror";
+import { useNavigation } from "./navigationContext";
+import {
+  ALL_FILES,
+  FilesActionsContext,
+  FilesStateContext,
+  parseView,
+  viewParam,
+  type FilesActions,
+  type FilesState,
+  type FilesView,
+} from "./filesContext";
+
+// Owns the Files module's shared state — the header search term and the view the rail picked (every
+// file, shared with me, or a folder) — so the header's search box, the FilesRail and the Files page
+// read it via hooks. Shaped like PlaylistsProvider: split State + Actions contexts, composed in
+// main.tsx around <App/>.
+//
+// The search is ephemeral (resets on reload, like the other list filters); the view is remembered per
+// account and mirrored to `?folder=` on the Files page, so a reload or a bookmark opens the same folder.
+// A `?folder=` in the address wins over the remembered one. A folder that no longer exists is the
+// page's to fall back from (it knows the folder list).
+
+function initialView(): FilesView {
+  return (
+    parseView(new URLSearchParams(window.location.search).get("folder")) ??
+    parseView(getAccountRaw(LS.filesView)) ??
+    ALL_FILES
+  );
+}
+
+export function FilesProvider({ children }: { children: ReactNode }) {
+  const [search, setSearchState] = useState("");
+  const [view, setViewState] = useState<FilesView>(initialView);
+
+  const setSearch = useCallback((q: string) => setSearchState(q), []);
+  const setView = useCallback((next: FilesView) => {
+    setViewState(next);
+    setAccountRaw(LS.filesView, viewParam(next));
+  }, []);
+
+  const { page } = useNavigation();
+  useUrlMirror({ folder: page === "files" ? viewParam(view) : null });
+
+  const state = useMemo<FilesState>(() => ({ search, view }), [search, view]);
+  const actions = useMemo<FilesActions>(() => ({ setSearch, setView }), [setSearch, setView]);
+
+  return (
+    <FilesActionsContext.Provider value={actions}>
+      <FilesStateContext.Provider value={state}>{children}</FilesStateContext.Provider>
+    </FilesActionsContext.Provider>
+  );
+}