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/OnboardingWizard.tsx

@@ -87,6 +87,7 @@ export default function OnboardingWizard({ me, onClose }: { me: Me; onClose: ()
     meta: { errorShownInline: true },
   });
 
+  const { mutate: startImport } = importSubs;
   // First time we see read access with zero imported channels, pull the user's
   // subscriptions automatically so they don't have to hunt for a button.
   useEffect(() => {
@@ -97,10 +98,11 @@ export default function OnboardingWizard({ me, onClose }: { me: Me; onClose: ()
       !importTriggered.current
     ) {
       importTriggered.current = true;
-      importSubs.mutate();
+      startImport();
     }
-    // eslint-disable-next-line react-hooks/exhaustive-deps
-  }, [me.can_read, myStatus.data?.channels_total]);
+    // `mutate` is stable (react-query memoises it) while the mutation object around it is not, so
+    // the effect can name what it calls. The ref keeps it to once regardless.
+  }, [me.can_read, myStatus.data, startImport]);
 
   // Closing without finishing the essential read step suppresses future auto-popups
   // (it stays reachable from Settings). Once read is granted, closing is just "done".