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/downloadRow.tsx

@@ -1,4 +1,4 @@
-import { useEffect, useState, type ReactNode } from "react";
+import { useState, type ReactNode } from "react";
 import { useTranslation } from "react-i18next";
 import { CalendarClock, Copy, File, Link2, Scissors } from "lucide-react";
 import clsx from "clsx";
@@ -77,8 +77,11 @@ function Thumb({ job }: { job: DownloadJob }) {
   const poster = job.poster_url ? api.posterImageUrl(job.id) : null;
   const primary = poster || remote;
   // On load failure (a genuinely missing poster file) fall back to the remote source thumbnail.
-  const [src, setSrc] = useState(primary);
-  useEffect(() => setSrc(primary), [primary]);
+  // Remembered BY the URL that failed, not as a value an effect re-syncs: these rows are recycled
+  // down a long virtualized list, so a boolean-style mirror painted the previous job's fallback
+  // over the next one's poster for a frame. See ui/Avatar, which had the same defect.
+  const [failedPrimary, setFailedPrimary] = useState<string | null>(null);
+  const src = failedPrimary && failedPrimary === primary ? remote : primary;
   // A generic uploaded file (media_kind='file') has no video frame to poster — it's a document/
   // archive, not playable media. Show a neutral file glyph instead of an (always-empty) thumbnail.
   if (job.asset?.media_kind === "file") {
@@ -97,7 +100,7 @@ function Thumb({ job }: { job: DownloadJob }) {
           loading="lazy"
           className="w-full h-full object-cover"
           onError={() => {
-            if (remote && src !== remote) setSrc(remote);
+            if (remote && src !== remote) setFailedPrimary(primary);
           }}
         />
       ) : null}