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/BootGate.tsx

@@ -1,4 +1,4 @@
-import { useLayoutEffect, useState, type ReactNode } from "react";
+import { useLayoutEffect, useRef, useState, type ReactNode } from "react";
 import { useQuery } from "@tanstack/react-query";
 import { useTranslation } from "react-i18next";
 import { api, getActiveAccount, setActiveAccount } from "../lib/api";
@@ -83,23 +83,31 @@ export default function BootGate({ children }: { children: ReactNode }) {
   //   • Adopt the account's language, which follows the account across devices — the local copy only
   //     knows what this browser last showed.
   const [readyFor, setReadyFor] = useState<number | null | undefined>(undefined);
+  // `me` is read for the identity it produced, so it rides a ref: a background refetch of the same
+  // account must not re-run the adoption over a language the user has just changed. That rule used
+  // to live in a dep list that excluded `me`, which made the dep list load-bearing for correctness —
+  // and the failure mode of getting it wrong was a page-reload loop, not a stale value.
+  const meRef = useRef(me);
+  meRef.current = me;
   useLayoutEffect(() => {
     if (identity === undefined) return;
     if (readyFor !== undefined) {
-      // The tree was built for another identity — see the header. The pin stays as it is: the tree
-      // being torn down still runs its unmount saves before the page goes, and with the pin moved they
-      // would land under the new account's local keys. The reloaded page's first probe corrects it.
-      location.reload();
+      // Already adopted. Only a DIFFERENT identity reloads: the tree was built for another account —
+      // see the header. The pin stays as it is: the tree being torn down still runs its unmount saves
+      // before the page goes, and with the pin moved they would land under the new account's local
+      // keys. The reloaded page's first probe corrects it. Comparing rather than merely testing for
+      // "set" is what makes this effect idempotent, so a re-run for any other reason is a no-op
+      // instead of a reload.
+      if (readyFor !== identity) location.reload();
       return;
     }
-    if (me && getActiveAccount() !== me.id) setActiveAccount(me.id);
-    const lang = me?.preferences?.language;
+    const account = meRef.current;
+    if (account && getActiveAccount() !== account.id) setActiveAccount(account.id);
+    const lang = account?.preferences?.language;
     if (typeof lang === "string" && isSupported(lang) && lang !== i18n.language) setLanguage(lang);
+    // eslint-disable-next-line react-hooks/set-state-in-effect -- the adoption latch. It cannot be derived: the whole point is that the tree mounts only AFTER the pin and the language have been applied, and both are writes to things outside React (sessionStorage, i18n). A layout effect so the frame that first shows the app already has them.
     setReadyFor(identity);
-    // `me` is read for the identity it produced; a background refetch of the same account must not
-    // re-run the adoption over a language the user has just changed.
-    // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, [identity]);
+  }, [identity, readyFor]);
 
   // No answer at all (a network or 5xx failure on the very first setup or account probe — past the
   // one retry `req` already makes): say so, with a retry. An unanswered setup probe is NOT "signed