fix(privacy): wipe plaintext/PII localStorage caches on logout (SEC-1/2)

Several localStorage caches held decrypted message content or user PII and
survived a normal logout, leaving residue on a shared device (the search
index was already wiped; these were not):

- cinny_scheduled_messages_v1 - decrypted IContent.body of pending sends
- cinny_recent_searches_v1     - search query text
- cinny_recent_forward_targets_v1 - recent forward contact/room graph
- cinny_recent_gifs_v1 / cinny_recent_stickers_v1 - media the user sent
- navToActivePath<userId>       - per-space last-visited room paths
- (plus the translation cache added earlier)

Add a clear function per module and a single auditable clearPlaintextCaches()
aggregator, called from both logout paths (logoutClient + the server-forced
SessionLoggedOut handler) alongside the existing session/search-index wipes.
Unit-tested.

Deliberately NOT cleared (documented in the aggregator): unsent composer
drafts and the presence status message (preserved by product decision N98);
SDK sync/crypto store + io.lotus.* account data (reminders/bookmarks/notes),
already wiped by mx.clearStores(); low-sensitivity UI/metadata residue.

The forward-targets/gifs/stickers/nav-path additions and the accurate
"not covered" documentation address findings from two review passes.

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

src/client/initMatrix.ts

@@ -7,7 +7,7 @@ import { LotusOidcTokenRefresher } from './oidcTokenRefresher';
 import { revokeOidcTokens } from './oidcLogout';
 import { pushSessionToSW } from '../sw-session';
 import { deleteSearchCacheDatabase } from '../app/utils/searchCache';
-import { clearTranslationCache } from '../app/state/translation';
+import { clearPlaintextCaches } from '../app/state/plaintextCaches';
 
 // Thrown when the local IndexedDB has a higher schema version than this SDK expects.
 // This happens after a downgrade (e.g. matrix-js-sdk was briefly upgraded and then reverted).
@@ -124,8 +124,10 @@ export const logoutClient = async (mx: MatrixClient) => {
   // The opt-in local search index stores decrypted plaintext — always wipe it
   // on logout. (clearLoginData below nukes all IDB databases, covering it too.)
   await deleteSearchCacheDatabase();
-  // The message-translation cache also holds decrypted plaintext — wipe it too.
-  clearTranslationCache();
+  // Other localStorage caches also hold decrypted plaintext / PII (translation,
+  // scheduled messages, recent searches/forwards/gifs/stickers, nav paths) —
+  // wipe them too.
+  clearPlaintextCaches(mx.getUserId() ?? undefined);
   // Remove only the session credential keys, preserving user preferences and
   // unsent drafts (N98). The factory-reset path is clearLoginData() below.
   removeFallbackSession();