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/Scheduler.tsx

@@ -139,17 +139,19 @@ function JobRow({
   runDisabled: boolean;
 }) {
   const { t } = useTranslation();
-  const [editing, setEditing] = useState(false);
-  const [val, setVal] = useState(String(job.interval_minutes));
+  // The box shows the server's value and holds a draft only while it is being edited, so there is
+  // nothing for an effect to re-sync: `null` means "follow the job". The poll behind this page
+  // refreshes every few seconds, which is exactly when a mirror-plus-effect would have shown the
+  // previous value for a frame.
+  const [draft, setDraft] = useState<string | null>(null);
+  const editing = draft !== null;
+  const val = draft ?? String(job.interval_minutes);
   const next = secsUntil(job.next_run);
-  useEffect(() => {
-    if (!editing) setVal(String(job.interval_minutes));
-  }, [job.interval_minutes, editing]);
 
   function save() {
     const m = parseInt(val, 10);
     if (Number.isFinite(m) && m >= 1 && m <= 1440 && m !== job.interval_minutes) onSave(m);
-    setEditing(false);
+    setDraft(null);
   }
 
   return (
@@ -170,10 +172,10 @@ function JobRow({
                 max={1440}
                 value={val}
                 autoFocus
-                onChange={(e) => setVal(e.target.value)}
+                onChange={(e) => setDraft(e.target.value)}
                 onKeyDown={(e) => {
                   if (e.key === "Enter") save();
-                  if (e.key === "Escape") setEditing(false);
+                  if (e.key === "Escape") setDraft(null);
                 }}
                 className="w-16 bg-surface border border-border rounded-sm px-1.5 py-0.5 text-xs focus:outline-hidden focus:ring-2 focus:ring-accent/40"
               />
@@ -187,7 +189,7 @@ function JobRow({
                 <Check className="w-3.5 h-3.5" />
               </FocusSafeButton>
               <button
-                onClick={() => setEditing(false)}
+                onClick={() => setDraft(null)}
                 className="text-muted hover:text-fg"
                 aria-label={t("common.cancel")}
               >
@@ -197,7 +199,7 @@ function JobRow({
           ) : (
             <Tooltip hint={t("scheduler.editInterval")}>
               <button
-                onClick={() => setEditing(true)}
+                onClick={() => setDraft(String(job.interval_minutes))}
                 className="group text-muted font-normal hover:text-accent inline-flex items-center gap-1 transition"
               >
                 · {t("scheduler.everyMin", { count: job.interval_minutes })}
@@ -256,12 +258,16 @@ function MaintenanceSettings({
   saving: boolean;
 }) {
   const { t } = useTranslation();
-  const [val, setVal] = useState(String(m.revalidate_batch));
-  useEffect(() => setVal(String(m.revalidate_batch)), [m.revalidate_batch]);
+  // Same shape as the job row above: a draft while editing, the server's value otherwise.
+  const [draft, setDraft] = useState<string | null>(null);
+  const val = draft ?? String(m.revalidate_batch);
 
   function save() {
     const n = parseInt(val, 10);
     if (Number.isFinite(n) && n >= m.min && n <= m.max && n !== m.revalidate_batch) onSave(n);
+    // Dropping the draft is what lets the box follow the server again once the save lands — and what
+    // clears a value that failed validation instead of leaving it stuck in the field.
+    setDraft(null);
   }
   const dirty = String(m.revalidate_batch) !== val;
 
@@ -281,7 +287,7 @@ function MaintenanceSettings({
           max={m.max}
           step={1000}
           value={val}
-          onChange={(e) => setVal(e.target.value)}
+          onChange={(e) => setDraft(e.target.value)}
           onKeyDown={(e) => e.key === "Enter" && save()}
           aria-label={t("scheduler.maintenance.batchLabel")}
           className="w-28 bg-surface border border-border rounded-sm px-2 py-1 text-sm tabular-nums focus:outline-hidden focus:ring-2 focus:ring-accent/40"