fix(frontend): four state defects the deep-review found in the E27 S2 sweep

Every one of them is the same shape: a value derived correctly, and a WRITE to
the thing it derives from left unattended. Deriving hides a stale value; it does
not consume it.

PlayerModal — the watch toggle inherited the previous video's auto-watch. Moving
the queue-advance sync into render was right for `currentVideoId`, but the same
block also synced `status`, and that one had an ordering constraint of its own:
useYtProgress's cleanup for the OUTGOING item can still call setWatched(true)
near the end of a clip, and cleanups run after the commit. As an effect the sync
landed last and corrected it; from render it lands first and the stray write
wins. Press Next in the final ten seconds and the NEXT video showed as watched —
one click on the toggle then sent onState(next, "new") for an item already new.
Fixed by keying the mirror to the id it describes, so a write for another item is
ignored rather than displayed, and no ordering has to be argued about at all.

ChannelsProvider — a focus-channel intent was masked by the page rather than
consumed, and nothing ever cleared it. The manager is a page chunk: it unmounts
on every navigation, and its seed effect runs again on each mount, so returning
to it re-applied a channel name from earlier in the session over whatever the
user had typed since. The consumed marker now lives in the provider, keyed on the
token — which also closes the case the old clear-on-leave effect never handled,
opening a channel page (that does not change `page` at all).

TagFilterChips + ChannelTags x2 — `open = openRaw && hidden > 0` hides the
overflow popover when the lane stops overflowing, but floating-ui only reports a
close for a real event on a mounted surface, so the raw flag stayed true and the
menu came back by itself on the next re-fit — a window resize or the text-size
slider is enough, with nobody near it. The flag is now reset in render against
the same condition that hides it.

Avatar + downloadRow — remembering a failed image BY URL fixed the recycled-row
frame but never forgot it, so a transient 429 pinned a channel to its initial for
the life of the instance, never re-requesting. The memory is now dropped once
`src` moves off it, which keeps both properties.

And playerModalHooks' last exhaustive-deps suppression claimed "nothing in the
body reads a render value" while reading five. The invariant that is actually
load-bearing is that each of them is a per-item snapshot — written down instead.

All four fixes are mutation-checked: each new test was seen red, against the
fixed line only, naming the real defect.

frontend/src/components/ChannelsProvider.dom.test.tsx

@@ -8,8 +8,8 @@
 // useAccountPersistedState / useSortState / useFilterState all resolve their account-scoped keys; only
 // navigationContext (the sole runtime dependency) is mocked, pinned to the channels page.
 import { describe, it, expect, beforeEach, vi } from "vitest";
-import { renderHook, act } from "@testing-library/react";
-import type { ReactNode } from "react";
+import { renderHook, act, render as renderRoot } from "@testing-library/react";
+import { useEffect, useRef, type ReactNode } from "react";
 
 vi.mock("./navigationContext", () => ({
   useNavigation: () => ({ page: "channels" }),
@@ -127,3 +127,85 @@ describe("ChannelsProvider — S8 shareable URL", () => {
     });
   });
 });
+
+// --- the focus-a-channel intent is consumed, not masked -------------------------------------------
+// `focusChannel` sends the user to the manager with its name filter seeded. The manager is a page
+// chunk: it unmounts on every navigation away, and on opening a channel page (which does not even
+// change `page`), and its seeding effect runs again on each mount. So the "already applied" marker
+// has to live in the PROVIDER — inside the page it would reset with the page and re-apply an intent
+// from earlier in the session over whatever the user has typed into the search box since.
+describe("ChannelsProvider — the focus-a-channel intent", () => {
+  /** Stands in for Channels.tsx:287-297 — the same effect, the same consume call. */
+  function Manager() {
+    const { focusName, focusToken } = useChannels();
+    const { setSearch, consumeFocus } = useChannelsActions();
+    const nameRef = useRef(focusName);
+    nameRef.current = focusName;
+    useEffect(() => {
+      const name = nameRef.current;
+      if (!name) return;
+      setSearch(name);
+      consumeFocus();
+    }, [focusToken, setSearch, consumeFocus]);
+    return null;
+  }
+
+  function Harness({ mounted }: { mounted: boolean }) {
+    return <>{mounted && <Manager />}</>;
+  }
+
+  it("seeds the search box once, and a later visit does not re-seed it", () => {
+    let api!: ReturnType<typeof useChannels> & ReturnType<typeof useChannelsActions>;
+    function Probe() {
+      api = { ...useChannels(), ...useChannelsActions() };
+      return null;
+    }
+    const { rerender } = renderRoot(
+      <ChannelsProvider>
+        <Probe />
+        <Harness mounted />
+      </ChannelsProvider>,
+    );
+
+    act(() => api.focusChannel("Kurzgesagt"));
+    expect(api.search).toBe("Kurzgesagt");
+
+    // The user looks for something else instead.
+    act(() => api.setSearch("veritasium"));
+
+    // Leave the manager (the page chunk unmounts) and come back.
+    rerender(
+      <ChannelsProvider>
+        <Probe />
+        <Harness mounted={false} />
+      </ChannelsProvider>,
+    );
+    rerender(
+      <ChannelsProvider>
+        <Probe />
+        <Harness mounted />
+      </ChannelsProvider>,
+    );
+
+    expect(api.search).toBe("veritasium");
+  });
+
+  it("re-seeds when the channel is focused again — the token is the intent", () => {
+    let api!: ReturnType<typeof useChannels> & ReturnType<typeof useChannelsActions>;
+    function Probe() {
+      api = { ...useChannels(), ...useChannelsActions() };
+      return null;
+    }
+    renderRoot(
+      <ChannelsProvider>
+        <Probe />
+        <Harness mounted />
+      </ChannelsProvider>,
+    );
+
+    act(() => api.focusChannel("Kurzgesagt"));
+    act(() => api.setSearch("veritasium"));
+    act(() => api.focusChannel("Kurzgesagt")); // the same channel, a new intent
+    expect(api.search).toBe("Kurzgesagt");
+  });
+});