refactor(frontend): the long tail — and eslint-suppressions.json is gone (E27 S2)

Closes the burn-down. Hook suppressions 113 -> 34, `exhaustive-deps` 45 -> 2 (the target was
<10), and every one of the 34 survivors is an inline directive NAMING the invariant it
protects. The suppressions file is deleted, because a bulk file hides the next violation
instead of answering it — and it cannot hold a warning-level rule at all.

The sprint plan's three verdicts missed the answer that turned out to be the most common one:
adjust the state DURING RENDER. React re-runs the component and discards that pass rather than
committing it, so the stale frame an effect-based reset paints does not exist. Applied to the
Plex player's up-next card, the playlist view reset, the feed's optimistic overrides, the
channel banner's retry count, the download center's tab guard, the Plex search highlight, the
comfort-pin latch, Welcome's per-view outcome and the card pager's page clamp. It is legal only
for a component's own state — PlexBrowse's language commit resets PlexProvider's filters, so
that one stays an effect and says why.

Mirrors of a prop or query became derived values or `draft ?? server`:
- `downloadRow` had ui/Avatar's defect exactly — a boolean "poster failed" that an effect reset,
  in rows recycled down a virtualized list, so one job's missing poster painted over the next
  one's for a frame. Keyed by URL now.
- Scheduler's two number editors held a mirror an effect re-synced, on a page that POLLS — the
  shape a poll can overwrite mid-edit. A draft while editing, the server's value otherwise; six
  tests where there were none, the one behavioural change (the save button stops being dirty at
  commit rather than at the echo) mutation-checked.
- SetupWizard's "no token" and WatchPage's "gone" are facts about the URL, so they are seeded and
  derived rather than pushed into state by an effect that had to commit first.

Elsewhere: PlexInfo's rating chip was a component declared during render — a new component type
every render, so React remounted the subtree and dropped the focus on its filter button. Hoisted.
Feed's undo action and PlexPlayer's session reclaim both re-entered their own useCallback, which
the checker reads as a forward reference; both go through the ref idiom this file already uses.
BootGate's dep list was load-bearing for correctness — a re-run meant `location.reload()` — so
the "already adopted THIS identity" comparison now enforces it in code, with a test that fails
without it (the existing reload tests all install their spy too late to catch it).

One correction to the batch before this: keying ChatThread's and Messages' decrypt passes on
`q.data` instead of `q.dataUpdatedAt` was wrong. react-query's structural sharing makes it safe
in production, but the component must not melt down when something hands back a fresh object per
render — and Messages.keyboard.dom.test does exactly that, so the effect span the suite. Both are
back on the fetch as the trigger with the payload behind a ref, which is honest AND loop-proof;
the replaced comment had warned about precisely this.

CLAUDE.md gains four canonical-patterns rows for the above, and its "React 18" line is now true.

frontend/src/components/ChatThread.tsx

@@ -42,6 +42,10 @@ export default function ChatThread({
     { intervalMs: POLL_MS, enabled: !needsKey },
   );
   const items = q.data?.items ?? [];
+  // The two effects below take the FETCH as their trigger and read the payload through this ref, so
+  // their dep lists name everything they touch without making the identity of `q.data` a trigger.
+  const threadRef = useRef(q.data);
+  threadRef.current = q.data;
 
   // Decrypt the thread when fresh server data arrives. Keyed on dataUpdatedAt (not `items`): the
   // `?? []` fallback is a new array reference every render, which would otherwise re-run this
@@ -56,7 +60,7 @@ export default function ChatThread({
       // hasn't re-set-up, so has no key) degrades to "can't decrypt" instead of rejecting the pass.
       let pub: string | null = null;
       let refreshed = false;
-      for (const m of q.data?.items ?? []) {
+      for (const m of threadRef.current?.items ?? []) {
         if (!m.ciphertext || !m.iv) continue;
         try {
           if (!pub) pub = await partnerPub(partnerId);
@@ -84,7 +88,8 @@ export default function ChatThread({
     return () => {
       cancelled = true;
     };
-    // eslint-disable-next-line react-hooks/exhaustive-deps
+    // `dataUpdatedAt` is the trigger and the payload is read through the ref above — see Messages,
+    // where keying this family on `q.data` itself produced a render loop.
   }, [q.dataUpdatedAt, ready, isSystem, partnerId, t]);
 
   // Opening the thread (or a new incoming message arriving) marks messages read server-side, so
@@ -101,13 +106,12 @@ export default function ChatThread({
       lastPartner.current = partnerId;
       lastIncoming.current = -1;
     }
-    const incoming = (q.data?.items ?? []).filter((m) => m.recipient_id === meId).length;
+    const incoming = (threadRef.current?.items ?? []).filter((m) => m.recipient_id === meId).length;
     if (incoming !== lastIncoming.current) {
       lastIncoming.current = incoming;
       qc.invalidateQueries({ queryKey: qk.conversations() });
       qc.invalidateQueries({ queryKey: qk.messageUnread() });
     }
-    // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [q.dataUpdatedAt, partnerId, meId, qc]);
 
   // Scroll to the newest message when the count changes (open / new message) — NOT on `plain`,