feat(search): opt-in persistent index for encrypted-room search (P4-8)

Raw-IndexedDB cache (lotus-search-cache: messages keyed [roomId,eventId] +
per-room coverage) merged into local search with in-memory-wins dedupe. OPT-IN
(default off) via a standalone atom — stores decrypted text at rest, so it ships
with a privacy note, a Clear button, and an unconditional wipe on logout
(initMatrix). All IDB errors degrade to cache-miss. +8 tests (1 IDB skip in node).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/state/searchCacheEnabled.ts

@@ -0,0 +1,24 @@
+import {
+  atomWithLocalStorage,
+  getLocalStorageItem,
+  setLocalStorageItem,
+} from './utils/atomWithLocalStorage';
+
+const SEARCH_CACHE_ENABLED = 'searchCacheEnabled';
+
+/**
+ * P4-8 — persistent encrypted-search cache opt-in flag (default `false`).
+ *
+ * Standalone, `localStorage`-backed boolean atom kept separate from
+ * `state/settings.ts` on purpose. When `true`, encrypted-room search persists a
+ * decrypted plaintext index to IndexedDB (`lotus-search-cache`) so coverage
+ * survives reloads. Because this writes decrypted plaintext at rest it must be
+ * explicitly opted into; the cache is clearable from the search UI and wiped on
+ * logout. Toggling this atom off stops all reads/writes but does NOT wipe
+ * existing data — that is the explicit "Clear cached index" button / logout.
+ */
+export const searchCacheEnabledAtom = atomWithLocalStorage<boolean>(
+  SEARCH_CACHE_ENABLED,
+  (key) => getLocalStorageItem<boolean>(key, false),
+  (key, value) => setLocalStorageItem(key, value),
+);