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/Channels.tsx

@@ -86,6 +86,7 @@ export default function Channels() {
     setLayout,
     setSort,
     focusChannel: onFocusChannel,
+    consumeFocus,
   } = useChannelsActions();
   const { openChannel: onViewChannel, setPage } = useNavigationActions();
   const feedFilters = useFeedFilters();
@@ -284,12 +285,19 @@ export default function Channels() {
   // The TOKEN is the intent (it advances even when the same channel is re-focused); the name is the
   // payload, so it rides a ref. Naming it would re-seed the box on any render that changed it,
   // overwriting whatever the user had since typed.
+  //
+  // `consumeFocus` is what stops this page's OWN remounts from re-applying it: this effect runs on
+  // every mount, and this page unmounts on every navigation away — and on opening a channel page,
+  // which does not even change `page`. The provider then reports `focusName` as null, so a later
+  // visit leaves the box exactly as the user left it.
   const focusChannelNameRef = useRef(focusChannelName);
   focusChannelNameRef.current = focusChannelName;
   useEffect(() => {
     const name = focusChannelNameRef.current;
-    if (name) setSearch(name);
-  }, [focusChannelToken, setSearch]);
+    if (!name) return;
+    setSearch(name);
+    consumeFocus();
+  }, [focusChannelToken, setSearch, consumeFocus]);
   // The header's reset intent clears the search box (skip the initial mount); the tag chips ride in
   // `filters.tags`, which goToFullHistory already clears by setting a fresh map.
   const resetRef = useRef(filtersResetToken);