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/state/threadNotifications.ts

@@ -3,15 +3,14 @@ import { ClientEvent, MatrixClient, MatrixEvent } from 'matrix-js-sdk';
 import { useEffect } from 'react';
 import { AccountDataEvent } from '../../types/matrix/accountData';
 import { ThreadNotificationsContent } from '../utils/threadNotifications';
+import { getAccountData } from '../utils/accountData';
 
 // Holds the parsed `io.lotus.thread_notifications` account data. Seeded and
 // kept in sync by `useBindThreadNotificationsAtom`.
 export const threadNotificationsAtom = atom<ThreadNotificationsContent>({});
 
 const readContent = (mx: MatrixClient): ThreadNotificationsContent =>
-  ((mx as any).getAccountData(AccountDataEvent.LotusThreadNotifications)?.getContent() as
-    | ThreadNotificationsContent
-    | undefined) ?? {};
+  getAccountData<ThreadNotificationsContent>(mx, AccountDataEvent.LotusThreadNotifications) ?? {};
 
 export const useBindThreadNotificationsAtom = (
   mx: MatrixClient,