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/ui/Avatar.dom.test.tsx

@@ -43,6 +43,9 @@ describe("Avatar · a failed image must not follow the component to the next ite
     expect(painted[0]).not.toContain(">Z<");
   });
 
+  // CHARACTERIZATION: the old boolean-plus-effect passes this too (the rerender is wrapped in act(),
+  // so its reset effect has run by the time the src is read). The frame-level guard is the test
+  // above, which the old shape fails; the retry guard is the test below, which it also fails.
   it("keeps the fallback for the URL that actually failed, not for whatever is current", () => {
     const { rerender } = render(
       <Avatar src="https://cdn.test/a.jpg" alt="avatar" fallback="Zed" />,
@@ -56,6 +59,23 @@ describe("Avatar · a failed image must not follow the component to the next ite
     );
   });
 
+  it("retries a URL it comes back to — a 429 is transient, and so is the memory of it", () => {
+    // The instance is not remounted here on purpose: the player's channel avatar and the Messages
+    // thread header both swap `src` on a live instance. A memory that outlived the reason for it
+    // would pin a throttled channel to its initial for the rest of the session.
+    const { rerender } = render(
+      <Avatar src="https://cdn.test/a.jpg" alt="avatar" fallback="Zed" />,
+    );
+    fireEvent.error(screen.getByRole("img", { name: "avatar" }));
+    expect(screen.getByText("Z")).toBeTruthy();
+
+    rerender(<Avatar src="https://cdn.test/b.jpg" alt="avatar" fallback="Zed" />);
+    rerender(<Avatar src="https://cdn.test/a.jpg" alt="avatar" fallback="Zed" />);
+    expect(screen.getByRole("img", { name: "avatar" }).getAttribute("src")).toBe(
+      "https://cdn.test/a.jpg",
+    );
+  });
+
   it("falls back when there is no src at all", () => {
     render(<Avatar src={null} alt="avatar" fallback="Zed" />);
     expect(screen.queryByRole("img")).toBeNull();