fix(compiler): React Compiler safety triage — 5 categories cleared

Cleared 4 rule buckets (37 violations, including 5 real bugs) and
silenced 1 informational bucket from the Next 16 / react-hooks v7
upgrade. Cleared rules promoted from `warn` back to `error` so new
regressions block CI.

Real bug fixes:
- `interest-contact-log-tab.tsx`: `useMemo` used for side effects
  (5 setState calls inside a memo body); converted to `useEffect`.
- `PieChart.tsx`: cumulative `let angle` mutation in a render-phase
  `map`; converted to `reduce` so the slice array is built without
  re-assignment.
- `documents-hub.tsx`: `useMemo(() => ({ count: 0 }))` used as a
  mutable drag counter; converted to `useRef`.
- `notes-list.tsx`: `Date.now()` read during render for note-edit
  countdown (impure) → pinned to a `now` state ticked every 30s.
- `onboarding-checklist.tsx` / `user-profile.tsx` /
  `user-settings.tsx`: `useEffect(() => void load(), [])` with the
  `load` function declared AFTER the effect — relied on hoisting,
  trips Compiler's "access before declared" rule. Declared inside
  the effect.

Pattern fixes (intentional cache-via-ref → state or layout-effect):
- 6 `ref.current = x` writes during render moved into layout
  effects (`use-realtime-invalidation`, `settings-form-card`,
  `inbox`).
- 3 `ref.current` reads during render (search totals cache,
  scanner file ref) rewritten to backed-by-state.
- `use-is-mobile.ts` rewritten on `useSyncExternalStore` to avoid
  the SSR-then-rehydrate setState dance.
- `use-notifications.ts` rewritten to write socket pushes directly
  into the React Query cache via `setQueryData`, removing a local
  state mirror.

Rule config (`eslint.config.mjs`):
- `react-hooks/purity` → error (was warn, cleared)
- `react-hooks/set-state-in-render` → error (was warn, cleared)
- `react-hooks/immutability` → error (was warn, cleared)
- `react-hooks/refs` → error (was warn, cleared)
- `react-hooks/incompatible-library` → off (informational only)
- `react-hooks/set-state-in-effect` → warn (51 remaining, all the
  useEffect→fetch→setState data-fetch pattern; migration to
  useQuery tracked in BACKLOG)

Verified: tsc clean, eslint 0 errors / 69 warnings (down from 105),
vitest 1315/1315, next build green.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/hooks/use-is-mobile.ts

@@ -1,29 +1,35 @@
 'use client';
 
-import { useEffect, useState } from 'react';
+import { useSyncExternalStore } from 'react';
 
 const MOBILE_QUERY = '(max-width: 1023.98px)';
 
+function subscribe(callback: () => void): () => void {
+  const mq = window.matchMedia(MOBILE_QUERY);
+  mq.addEventListener('change', callback);
+  return () => mq.removeEventListener('change', callback);
+}
+
+function getSnapshot(): boolean {
+  return window.matchMedia(MOBILE_QUERY).matches;
+}
+
+function getServerSnapshot(): boolean {
+  // Server has no window — default to desktop. Client hydrates to the
+  // true viewport state without a flash because useSyncExternalStore
+  // is React 18's "this is server-mismatch safe" primitive.
+  return false;
+}
+
 /**
  * Returns true when the viewport is below the `lg` Tailwind breakpoint.
- * Backed by a media-query listener; safe to call from any client component.
- * Server renders return `false` (desktop default) - clients hydrate to the
- * true viewport state on mount.
+ * Backed by useSyncExternalStore so render reads stay pure (no
+ * useEffect → setState cascade); React Compiler-safe.
  *
  * Not unit-tested: the repo's vitest is configured for environment='node'
  * (no @testing-library/react / DOM env). Verified through the mobile-shell
  * Playwright visual snapshots in Task 23.
  */
 export function useIsMobile(): boolean {
-  const [isMobile, setIsMobile] = useState(false);
-
-  useEffect(() => {
-    const mq = window.matchMedia(MOBILE_QUERY);
-    const update = (e: { matches: boolean }) => setIsMobile(e.matches);
-    setIsMobile(mq.matches);
-    mq.addEventListener('change', update);
-    return () => mq.removeEventListener('change', update);
-  }, []);
-
-  return isMobile;
+  return useSyncExternalStore(subscribe, getSnapshot, getServerSnapshot);
 }