feat(player): the YouTube player and the file viewer are named in the address

The player names what it plays in its own history entry (watch, wrel, wtab): a reload or a
bookmark reopens it on the page that hosts it, with that page's list as the queue, or alone
from the catalog (GET /api/videos/{id}/row); a page that hosts none gets the shell's. Back
lands on the page's address without it (useBackToClose's url; ui/Modal passes it on). The
Files viewer and player do the same (fview, fplay). The Plex and channel-manager views are
always written in full, as their fallback is remembered state.

frontend/src/components/ui/Modal.tsx

@@ -65,6 +65,7 @@ export default function Modal({
   placement = "center",
   initialFocus,
   headerActions,
+  url,
 }: {
   title: ReactNode;
   initialFocus: ModalFocus;
@@ -81,10 +82,14 @@ export default function Modal({
   placement?: "center" | "drawer" | "drawer-start" | "top";
   /** Controls beside the ✕ in the header (a side panel's pin and customize, C2). */
   headerActions?: ReactNode;
+  /** What the dialog names in the address while it is open, when it is part of the view (a viewer, a
+   *  player): written into its own history entry, so a reload or a bookmark reopens it and Back lands
+   *  on the address without it (lib/history). */
+  url?: Record<string, string | null>;
 }) {
   const { t } = useTranslation();
   // Browser/mouse Back closes the topmost modal instead of navigating away.
-  useBackToClose(onClose);
+  useBackToClose(onClose, url);
   // ONE container element for the dialog's whole life, re-parented between <body> and the fullscreen
   // element — and attached before the children commit, so an `autoFocus` child
   // still gets the focus. Both rules live in the hook; `ui/AnchoredPopover` shares it.