DP16: add typed get/setAccountData helpers, remove ~19 any-casts

Add centralized typed helpers in src/app/utils/accountData.ts:
- getAccountData<T>(mx, eventType): T | undefined (returns content)
- setAccountData<T>(mx, eventType, content): Promise<void>

These wrap the single `as any` cast needed because matrix-js-sdk's typed
overloads reject the fork's custom account-data event names. Every call site
now stays fully typed on its content shape.

Route all account-data reads/writes that previously used
`(mx as any).getAccountData/setAccountData` or `mx.getAccountData(... as any)`
through the helpers (or, where a MatrixEvent is needed, through the existing
utils/room.ts getAccountData whose param is widened to accept string keys).
No behavior change: same event types, same content shapes.

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

src/app/hooks/useBookmarks.ts

@@ -1,6 +1,7 @@
 import { useCallback, useEffect, useState } from 'react';
 import { ClientEvent, ClientEventHandlerMap, MatrixClient } from 'matrix-js-sdk';
 import { useMatrixClient } from './useMatrixClient';
+import { getAccountData, setAccountData } from '../utils/accountData';
 
 export type Bookmark = {
   roomId: string;
@@ -18,10 +19,7 @@ type BookmarksContent = {
 };
 
 function readBookmarks(mx: MatrixClient): Bookmark[] {
-  return (
-    (mx.getAccountData(BOOKMARKS_KEY as any)?.getContent() as BookmarksContent | undefined)
-      ?.bookmarks ?? []
-  );
+  return getAccountData<BookmarksContent>(mx, BOOKMARKS_KEY)?.bookmarks ?? [];
 }
 
 // Module-scoped serialization state.
@@ -85,7 +83,7 @@ function enqueueBookmarkWrite(
     const next = compute(state.latest);
     state.latest = next;
     state.listeners.forEach((listener) => listener(next));
-    await (mx as any).setAccountData(BOOKMARKS_KEY, { bookmarks: next });
+    await setAccountData(mx, BOOKMARKS_KEY, { bookmarks: next });
   });
   // Keep the chain alive even if one write rejects, but propagate the
   // rejection to this caller so it can react (e.g. retry).