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/lib/channelsUrlState.dom.test.ts

@@ -36,15 +36,31 @@ describe("channelsParamUpdates", () => {
     expect(channelsParamUpdates({ ...BASE, search: "  " }).cq).toBeNull();
   });
 
-  it("emits nothing for the all-default state", () => {
-    expect(channelsParamUpdates(BASE)).toEqual({ ctab: null, csort: null, cf: null, cq: null });
+  it("writes the default view in full: this browser remembers one a link must not inherit", () => {
+    expect(channelsParamUpdates(BASE)).toEqual({
+      ctab: "subscribed",
+      csort: "none",
+      cf: "",
+      cq: null,
+    });
+  });
+
+  it("a link to the default view opens it over another remembered one", () => {
+    const params = applyUpdates(channelsParamUpdates(BASE));
+    const back = channelsParamsToState(params, {
+      view: "hidden",
+      sort: { key: "subscribers", dir: "desc" },
+      filters: { sync: "fully" },
+      search: "",
+    });
+    expect(back).toEqual(BASE);
   });
 
   it("clears every key when passed null (off-manager page)", () => {
     expect(channelsParamUpdates(null)).toEqual({ ctab: null, csort: null, cf: null, cq: null });
   });
 
-  it("emits only non-default keys", () => {
+  it("writes the view picked", () => {
     const u = channelsParamUpdates({
       view: "hidden",
       sort: { key: "subscribers", dir: "desc" },
@@ -57,8 +73,8 @@ describe("channelsParamUpdates", () => {
     expect(u.cq).toBe("tom");
   });
 
-  it("drops empty filter entries so a cleared funnel emits no cf", () => {
-    expect(channelsParamUpdates({ ...BASE, filters: { tags: [], sync: "" } }).cf).toBeNull();
+  it("drops empty filter entries so a cleared funnel writes an empty cf", () => {
+    expect(channelsParamUpdates({ ...BASE, filters: { tags: [], sync: "" } }).cf).toBe("");
   });
 });