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/Messages.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useState } from "react";
+import { useEffect, useRef, useState } from "react";
 import { qk } from "../lib/queryKeys";
 import { useTranslation } from "react-i18next";
 import { useQuery } from "@tanstack/react-query";
@@ -85,15 +85,20 @@ function ConversationList({
   const items = q.data?.items ?? [];
   const [previews, setPreviews] = useState<Record<number, string>>({});
 
-  // Decrypt conversation previews when fresh data arrives. Keyed on dataUpdatedAt (not `items`):
-  // the `?? []` fallback is a new array each render, which would otherwise spin this effect
-  // (and setPreviews) every render while the query is loading.
+  // Decrypt conversation previews when fresh data arrives. The TRIGGER is `dataUpdatedAt` and the
+  // payload is read through a ref, which is what makes the dep list below complete without making
+  // the identity of `q.data` a trigger. That distinction is load-bearing: the `?? []` fallback is a
+  // new array every render, and anything handing back a fresh object per render (a stub, a caller
+  // that rebuilds it) turns "re-run when the data changes" into a render loop through setPreviews.
+  // Measured the hard way — E27 S2 keyed this on `q.data`, and Messages.keyboard.dom.test hung.
+  const conversationsRef = useRef(q.data);
+  conversationsRef.current = q.data;
   useEffect(() => {
     if (!ready) return;
     let cancelled = false;
     (async () => {
       const out: Record<number, string> = {};
-      for (const c of q.data?.items ?? []) {
+      for (const c of conversationsRef.current?.items ?? []) {
         const m = c.last_message;
         if (m.kind === "user" && m.ciphertext && m.iv) {
           try {
@@ -109,7 +114,6 @@ function ConversationList({
     return () => {
       cancelled = true;
     };
-    // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [q.dataUpdatedAt, ready]);
 
   function previewText(c: Conversation): string {