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/moduleRegistry.tsx

@@ -26,6 +26,8 @@ const Playlists = lazyWithPreload(() => import("./Playlists"));
 const PlexBrowse = lazyWithPreload(() => import("./PlexBrowse"));
 const PlexSidebar = lazyWithPreload(() => import("./PlexSidebar"));
 const NotificationsPanel = lazyWithPreload(() => import("./NotificationsPanel"));
+const Files = lazyWithPreload(() => import("./Files"));
+const FilesRail = lazyWithPreload(() => import("./FilesRail"));
 const DownloadCenter = lazyWithPreload(() => import("./DownloadCenter"));
 const Stats = lazyWithPreload(() => import("./Stats"));
 const Scheduler = lazyWithPreload(() => import("./Scheduler"));
@@ -54,6 +56,7 @@ export const PAGE_CONTENT: Record<Page, PageDef> = {
   playlists: { Component: Playlists, maxWidth: "64rem" },
   plex: { Component: PlexBrowse, maxWidth: "100rem" },
   notifications: { Component: NotificationsPanel, maxWidth: "48rem" },
+  files: { Component: Files, maxWidth: "80rem" },
   downloads: { Component: DownloadCenter, maxWidth: "56rem" },
   stats: { Component: Stats },
   scheduler: { Component: Scheduler },
@@ -73,8 +76,9 @@ type RailProps = {
   onToggleCollapse: () => void;
 };
 
-// Which collapse flag a rail uses (feed + plex share the filter rail's flag; playlists has its own).
-type RailCollapseKey = "filter" | "playlists";
+// Which collapse flag a rail uses (feed + plex share the filter rail's flag; playlists and files
+// have their own).
+type RailCollapseKey = "filter" | "playlists" | "files";
 
 export type RailDef = {
   Rail: ComponentType<RailProps>;
@@ -83,11 +87,12 @@ export type RailDef = {
   collapse: RailCollapseKey;
 };
 
-// page id → its side rail (only the three pages that have one). App renders it in the rail slot.
+// page id → its side rail (only the pages that have one). App renders it in the rail slot.
 export const PAGE_RAIL: Partial<Record<Page, RailDef>> = {
   feed: { Rail: Sidebar, panelId: "feed", collapse: "filter" },
   plex: { Rail: PlexSidebar, panelId: "plex", collapse: "filter" },
   playlists: { Rail: PlaylistsRail, panelId: "playlists", collapse: "playlists" },
+  files: { Rail: FilesRail, panelId: "files", collapse: "files" },
 };
 
 /** Fetch the code of these pages and of their side rails now, ahead of their first visit. Errors are