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/features/message-search/MessageSearch.tsx

@@ -11,6 +11,8 @@ import {
   Line,
   toRem,
   Button,
+  Switch,
+  Chip,
 } from 'folds';
 import { useAtom, useAtomValue } from 'jotai';
 import { useVirtualizer } from '@tanstack/react-virtual';
@@ -41,7 +43,9 @@ import {
   ResultGroup,
   useMessageSearch,
 } from './useMessageSearch';
-import { useLocalMessageSearch } from './useLocalMessageSearch';
+import { LocalSearchResult, useLocalMessageSearch } from './useLocalMessageSearch';
+import { searchCacheEnabledAtom } from '../../state/searchCacheEnabled';
+import { clearAll as clearSearchCache } from '../../utils/searchCache';
 import { addRecentSearch, recentSearchesAtom } from '../../state/recentSearches';
 import { SearchResultGroup } from './SearchResultGroup';
 import { SearchInput } from './SearchInput';
@@ -240,6 +244,10 @@ export function MessageSearch({
   // Bump this whenever more messages are loaded so localResult re-computes
   const [cacheVersion, setCacheVersion] = useState(0);
   const handleCacheLoaded = useCallback(() => setCacheVersion((v) => v + 1), []);
+  // Explicit wipe of the persistent on-disk index, then re-run the merge.
+  const handleClearSearchCache = useCallback(() => {
+    clearSearchCache().then(() => setCacheVersion((v) => v + 1));
+  }, []);
 
   // The rooms actually in scope for this search (mirrors server-side logic)
   const localSearchRooms = useMemo(
@@ -253,24 +261,43 @@ export function MessageSearch({
   const hasActiveSearch = msgSearchParams.term !== undefined || !!msgSearchParams.senders?.length;
   const senderOnlyMode = !msgSearchParams.term && !!msgSearchParams.senders?.length;
 
-  // Run synchronous client-side search immediately.
+  // Run the client-side search whenever inputs change.
   // In text-search mode: covers encrypted rooms only (server handles plaintext).
   // In sender-only mode: covers all rooms (server has no sender-only search).
-  // cacheVersion in deps so it re-runs after "Load more" paginates new events.
-  const localResult = useMemo(() => {
-    if (!hasActiveSearch) return null;
-    return searchLocalMessages({
+  // The scan is async because — when the persistent cache is enabled — it also
+  // reads cached rows from IndexedDB and merges them with the in-memory hits.
+  // cacheVersion in deps so it re-runs after "Load more" paginates new events;
+  // searchCacheEnabled so toggling the cache re-runs the merge.
+  const [searchCacheEnabled, setSearchCacheEnabled] = useAtom(searchCacheEnabledAtom);
+  const [localResult, setLocalResult] = useState<LocalSearchResult | null>(null);
+  useEffect(() => {
+    if (!hasActiveSearch) {
+      setLocalResult(null);
+      return undefined;
+    }
+    let cancelled = false;
+    searchLocalMessages({
       term: msgSearchParams.term ?? '',
       roomIds: localSearchRooms,
       senders: msgSearchParams.senders,
+      fromTs: msgSearchParams.fromTs,
+      toTs: msgSearchParams.toTs,
+    }).then((result) => {
+      if (!cancelled) setLocalResult(result);
     });
-    // eslint-disable-next-line react-hooks/exhaustive-deps
+    return () => {
+      cancelled = true;
+    };
   }, [
     searchLocalMessages,
     localSearchRooms,
     msgSearchParams.term,
     msgSearchParams.senders,
+    msgSearchParams.fromTs,
+    msgSearchParams.toTs,
+    hasActiveSearch,
     cacheVersion,
+    searchCacheEnabled,
   ]);
 
   const { status, data, error, fetchNextPage, hasNextPage, isFetchingNextPage } = useInfiniteQuery({
@@ -668,6 +695,37 @@ export function MessageSearch({
                     ? `Showing locally cached messages from ${localResult.searchedRoomsCount} encrypted room${localResult.searchedRoomsCount !== 1 ? 's' : ''}. Load more history below to extend coverage.`
                     : `No matches in your local cache. Load messages below to search further back.`}
               </Text>
+              <Box
+                alignItems="Center"
+                gap="200"
+                style={{
+                  padding: config.space.S200,
+                  background: color.SurfaceVariant.Container,
+                  borderRadius: config.radii.R300,
+                }}
+              >
+                <Switch
+                  variant="Primary"
+                  value={searchCacheEnabled}
+                  onChange={setSearchCacheEnabled}
+                />
+                <Box grow="Yes" direction="Column" style={{ minWidth: 0 }}>
+                  <Text size="T300">Persist search index on this device</Text>
+                  <Text size="T200" priority="300">
+                    Stores decrypted text on this device
+                  </Text>
+                </Box>
+                {searchCacheEnabled && (
+                  <Chip
+                    variant="Secondary"
+                    radii="Pill"
+                    onClick={handleClearSearchCache}
+                    before={<Icon size="100" src={Icons.Delete} />}
+                  >
+                    <Text size="T200">Clear cached index</Text>
+                  </Chip>
+                )}
+              </Box>
               <Line size="300" variant="Surface" />
             </Box>
             {localGroups.length > 0 && (