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.tsx

@@ -183,7 +183,16 @@ export function ChannelsProvider({ children }: { children: ReactNode }) {
   // rather than cleared by an effect when you leave. focusChannel sets the name and navigates in the
   // same handler, so the value is live by the time channels renders; everywhere else it reads null,
   // one render earlier than the effect used to make it.
-  const focusName = page === "channels" ? focusNameRaw : null;
+  //
+  // But an intent has to be CONSUMED, not merely masked, and the consumed marker has to live HERE.
+  // Channels is a page chunk that unmounts on every navigation (and on opening a channel page, which
+  // does not even change `page`), so its seeding effect runs again on each remount. A marker inside
+  // it would reset with it and re-apply an intent from earlier in the session over whatever the user
+  // has since typed into the search box. Keyed on the token rather than the name, because
+  // re-focusing the same channel is a new intent.
+  const [focusApplied, setFocusApplied] = useState(0);
+  const consumeFocus = useCallback(() => setFocusApplied(focusToken), [focusToken]);
+  const focusName = page === "channels" && focusApplied !== focusToken ? focusNameRaw : null;
 
   // Label the entry the manager was REACHED on (setPage pushes a bare { sfPage }), so stepping back
   // onto it later restores the tab it belonged to rather than leaving whatever tab is current.
@@ -216,9 +225,19 @@ export function ChannelsProvider({ children }: { children: ReactNode }) {
       setLayout,
       setSort,
       focusChannel,
+      consumeFocus,
       goToFullHistory,
     }),
-    [setSearch, setFilters, setView, setLayout, setSort, focusChannel, goToFullHistory],
+    [
+      setSearch,
+      setFilters,
+      setView,
+      setLayout,
+      setSort,
+      focusChannel,
+      consumeFocus,
+      goToFullHistory,
+    ],
   );
 
   // S8: mirror the content state (tab + sort + column filters) into the address bar under the namespaced