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.tsx

@@ -25,7 +25,16 @@ export default function Avatar({
   // one avatar's 429 used to paint the fallback initial over the NEXT item's perfectly good image
   // for a frame — React committed the render with the new `src` and the stale `failed`, and only the
   // post-paint effect cleared it. Derived state cannot be stale, so there is nothing to reset.
+  //
+  // Forgetting the URL once `src` has moved off it is the other half, and it is what keeps the 429
+  // TRANSIENT: without it the memory outlives the reason for it, and a mounted instance whose `src`
+  // swaps — the player's channel avatar stepping through a queue, the Messages thread header — would
+  // show the initial for a throttled channel forever, never re-requesting even after the CDN
+  // relented. Written in render because it is convergent (the write falsifies its own condition) and
+  // because an effect would leave one committed frame in which the memory is stale, which is the
+  // frame this whole component is about.
   const [failedSrc, setFailedSrc] = useState<string | null>(null);
+  if (failedSrc !== null && failedSrc !== src) setFailedSrc(null);
   const failed = !!src && failedSrc === src;
 
   if (src && !failed) {