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/lib/playerModalHooks.ts

@@ -486,9 +486,17 @@ export function useYtProgress({
     };
     // ONE PLAYER PER VIDEO is the invariant, and it is why this dep list is deliberately just the
     // id: re-running would destroy and rebuild the YouTube iframe, losing the playing position and
-    // every listener attached to it. Nothing in the body reads a render value — the mutable state it
-    // needs mid-session (currentIdRef, modeRef, navRef, sessionPosRef, autoMarkedRef,
-    // errorStreakRef, playerRef) is all behind refs, which is the enforcement, not this comment.
+    // every listener attached to it.
+    //
+    // This body DOES capture render values — `resumeAt`, `setWatched`, `advanceOnEnd`,
+    // `reapplyStored`, `focusModal`, `qc` — and it must, because each of them is a PER-ITEM
+    // SNAPSHOT: they describe the video this player was built for, and re-reading them mid-session
+    // would be the bug, not the fix. `resumeAt` is the clearest case, since persist() writes
+    // sessionPosRef and so changes it while `activeId` has not moved; the player is already playing
+    // from it and must not be told again. What genuinely has to stay live mid-session is behind refs
+    // (currentIdRef, modeRef, navRef, sessionPosRef, autoMarkedRef, errorStreakRef, playerRef) — and
+    // THAT split is the enforcement: a value that is not a snapshot does not belong in this closure.
+    //
     // useEffectEvent cannot help here the way it does above: YT.Player takes its callbacks as an
     // object literal, and an effect event may not be passed around as a value.
     // eslint-disable-next-line react-hooks/exhaustive-deps