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/modules/channels/ChannelsProvider.dom.test.tsx

@@ -106,13 +106,13 @@ describe("ChannelsProvider — shareable URL", () => {
     expect(result.current.filters).toEqual({ sync: "needsFull" });
   });
 
-  it("mirrors the tab into the address bar and clears it back to the default", () => {
+  it("mirrors the tab into the address bar, the default one too", () => {
     const { result } = render();
     act(() => result.current.setView("blocked"));
     expect(new URLSearchParams(window.location.search).get("ctab")).toBe("blocked");
-    // Back to the default tab clears the key (only non-defaults are emitted).
+    // The default tab is named as well: a link without it would open the tab this browser remembers.
     act(() => result.current.setView("subscribed"));
-    expect(new URLSearchParams(window.location.search).has("ctab")).toBe(false);
+    expect(new URLSearchParams(window.location.search).get("ctab")).toBe("subscribed");
   });
 
   it("a PARTIAL shared link does not clobber the account's stored sort/filters", () => {